Nanowire-based transparent conductors and applications thereof

ABSTRACT

A transparent conductor including a conductive layer coated on a substrate is described. More specifically, the conductive layer comprises a network of nanowires that may be embedded in a matrix. The conductive layer is optically clear, patternable and is suitable as a transparent electrode in visual display devices such as touch screens, liquid crystal displays, plasma display panels and the like.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/712,096, filed Feb. 24, 2010 (allowed); which is acontinuation-in-part of U.S. patent application Ser. No. 11/871,767,filed Oct. 12, 2007 (pending); which claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application No. 60/851,652 filed Oct.12, 2006; U.S. Provisional Patent Application No. 60/911,058 filed Apr.10, 2007; and U.S. Provisional Patent Application No. 60/913,231 filedApr. 20, 2007. All of these applications are incorporated herein byreference in their entireties.

BACKGROUND

1. Technical Field

This invention is related to transparent conductors, methods ofmanufacturing and patterning the same, and applications thereof.

2. Description of the Related Art

Transparent conductors refer to thin conductive films coated onhigh-transmittance surfaces or substrates. Transparent conductors may bemanufactured to have surface conductivity while maintaining reasonableoptical transparency. Such surface conducting transparent conductors arewidely used as transparent electrodes in flat liquid crystal displays,touch panels, electroluminescent devices, and thin film photovoltaiccells, as anti-static layers and as electromagnetic wave shieldinglayers.

Currently, vacuum deposited metal oxides, such as indium tin oxide(ITO), are the industry standard materials to provide opticallytransparency and electrical conductivity to dielectric surfaces such asglass and polymeric films. However, metal oxide films are fragile andprone to damage during bending or other physical stresses. They alsorequire elevated deposition temperatures and/or high annealingtemperatures to achieve high conductivity levels. There also may beissues with the adhesion of metal oxide films to substrates that areprone to adsorbing moisture such as plastic and organic substrates, e.g.polycarbonates. Applications of metal oxide films on flexible substratesare therefore severely limited. In addition, vacuum deposition is acostly process and requires specialized equipment. Moreover, the processof vacuum deposition is not conducive to forming patterns and circuits.This typically results in the need for expensive patterning processessuch as photolithography.

Conductive polymers have also been used as optically transparentelectrical conductors. However, they generally have lower conductivityvalues and higher optical absorption (particularly at visiblewavelengths) compared to the metal oxide films, and suffer from lack ofchemical and long-term stability.

Accordingly, there remains a need in the art to provide transparentconductors having desirable electrical, optical and mechanicalproperties, in particular, transparent conductors that are adaptable toany substrates, and can be manufactured and patterned in a low-cost,high-throughput process.

BRIEF SUMMARY

Transparent conductors based on electrically conductive nanowires in anoptically clear matrix are described. The transparent conductors arepatternable and are suitable as transparent electrodes in a wide varietyof devices including, without limitation, display devices (e.g., touchscreens, liquid crystal displays, plasma display panels and the like),electroluminescent devices, and photovoltaic cells.

One embodiment describes an optically uniform transparent conductorcomprising: a substrate; a conductive film on the substrate, theconductive film including a plurality of interconnecting nanostructures,wherein a pattern on the conductive film defines (1) an unetched regionhaving a first resistivity, a first transmission and a first haze and(2) an etched region having a second resistivity, a second transmissionand a second haze; and wherein the etched region is less conductive thanthe unetched region, a ratio of the first resistivity over the secondresistivity is at least 1000; the first transmission differs from thesecond transmission by less than 5%; and the first haze differs from thesecond haze by less than 0.5%.

A further embodiment provides a process, which comprises: forming aconductive film comprising a plurality of interconnectingnanostructures; and etching the conductive film according to a patternto provide (1) an unetched region having a first resistivity, a firsttransmission and a first haze and (2) an etched region having a secondresistivity, a second transmission and a second haze; and wherein theetched region is less conductive than the unetched region, a ratio ofthe first resistivity over the second resistivity is at least 1000; thefirst transmission differs from the second transmission by less than 5%;and the first haze differs from the second haze by less than 0.5%.

Another embodiment describes a process, which comprises: forming aconductive film comprising a plurality of interconnectingnanostructures; etching the conductive film according to a pattern toprovide (1) an unetched region having a first intermediate resistivity,and (2) an etched region having a second intermediate resistivity,wherein a first ratio of the first intermediate resistivity over thesecond intermediate resistivity is less than 1000; and heating theconductive film such that the etched region has a first finalresistivity and the unetched region has a second final resistivity,wherein a second ratio of the first final resistivity over the secondfinal resistivity is at least 1000, and wherein the etched region andthe unetched region are optically uniform.

Yet another embodiment provides a process which comprises: forming aconductive film comprising a plurality of interconnectingnanostructures; and contacting an etchant solution with the conductivefilm according to a pattern to provide an unetched region and an etchedregion, wherein the unetched region and the etched region havesubstantially the same optical properties, and wherein the etchantsolution is an aqueous solution including a metal salt.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn are not intendedto convey any information regarding the actual shape of the particularelements, and have been selected solely for ease of recognition in thedrawings.

FIG. 1 is a schematic illustration of a nanowire.

FIG. 2 is a graph illustrating the expected optical properties of asilver nanoellipsoids at various wavelengths of light.

FIG. 3 illustrates the absorption spectrum of a silver nanowire layer ona polyethylene terephthalate (PET) substrate.

FIG. 4 is a graph illustrating expected values for various resistivityproperties of a nanowire based on the wire diameter.

FIG. 5 is a graph illustrating the expected overall resistivity as afunction of the diameters of nanowires.

FIG. 6 shows an SEM image of a single silver nanowires connectingbetween two metal contacts.

FIG. 7 illustrates a network of filamentous proteins that function asbiological templates for a transparent conductor.

FIG. 8 illustrates a protein scaffold coupled to conductive particlesvia various binding sites.

FIG. 9 illustrates the formation of a conductive network of biologicaltemplates based on the coupling of associate peptides.

FIG. 10A illustrates schematically an embodiment of a metalnanowires-based transparent conductor.

FIG. 10B illustrates schematically another embodiment of a metalnanowires-based transparent conductor.

FIG. 10C shows schematically a further embodiment of a metal nanowirebased transparent conductor in which portions of the nanowires areexposed on a surface of the transparent conductor.

FIG. 10D shows an SEM image of silver nanowires protruding out of asurface of the transparent conductor.

FIG. 10E illustrates schematically another embodiment of a metalnanowires-based transparent conductor.

FIG. 10F illustrates schematically a multi-layer structure having atransparent conductive layer coated with a surface-conductive overcoat.

FIG. 11 illustrates schematically a further embodiment of a metalnanowires-based transparent conductor having a multi-layer structure.

FIG. 12 shows a transparent conductor structure having a reservoir fordelivering a vapor phase inhibitor (VPI).

FIGS. 13A-13D show an example of a fabrication process of a transparentconductor.

FIG. 14A shows an example of a fabrication process of a transparentconductor by web coating.

FIG. 14B shows another example of a fabrication process of a transparentconductor by web coating.

FIG. 15A shows a web coating system and a flow process for fabricating atransparent conductor.

FIG. 15B shows an SEM image of a conductive layer following apost-treatment of pressure application.

FIGS. 16A-16C show an example of a lamination process.

FIGS. 17A-17C show another example of a lamination process.

FIG. 18 shows an example of photo-patterning a conductive layer.

FIGS. 19A-19B show an example of a continuous photo-patterning methodsuitable for a web coating process.

FIG. 20 shows a partial system and a process of fabricating a patternedtransparent conductor.

FIGS. 21A-21B are SEM images showing the conformality of ananowire-based transparent conductor film.

FIG. 22 is a graphic representation showing temperature resistance ofnanowire-based transparent conductor film.

FIG. 23 is a cross-sectional view of a switching device of a TFTbackplane according to one embodiment.

FIG. 24 shows a display device comprising transparent electrodes basedon metal nanowires.

FIG. 25 shows a cross-sectional view of a LCD based on a top-gate TFTaccording to one embodiment.

FIG. 26A shows a cross-sectional view of a color filter substrate in aTN mode LCD according to one embodiment.

FIG. 26B shows a cross-sectional view of a color filter substrate in anIPS mode LCD according to another embodiment.

FIG. 27 shows a cross-sectional view of a PDP according to oneembodiment.

FIG. 28 shows a touch screen device comprising two transparentconductors based on metal nanowires.

FIG. 28A shows two opposing transparent conductors coated with overcoatsin a touch screen device.

FIG. 29A shows a homojunction solar cell structure according to oneembodiment.

FIG. 29B shows a heterojunction solar cell structure according toanother embodiment.

FIG. 29C shows a multi-junction solar cell structure according toanother embodiment.

FIG. 30 shows a thin film EL device according to one embodiment.

FIG. 31 shows a typical release profile of H₂S gas from freshly cookedegg yolks.

FIG. 32A shows the light transmission of six samples of conductive filmsbefore and after an accelerated H₂S corrosion test.

FIG. 32B shows the resistance of six samples of conductive films beforeand after an accelerated H₂S corrosion test.

FIG. 32C shows the haze values of six samples of conductive films beforeand after an accelerated H₂S corrosion test.

FIG. 33 shows an example of directly patterning a nanowire-basedtransparent conductive film.

FIGS. 34A-34F show photographs of the patterned conductive films beforeand after an adhesive tape treatment at various levels of magnification.

FIGS. 35A-35D show photographs of another exemplary conductive filmbefore and after a solvent treatment.

FIGS. 36A-36C show a progression of etching and a final pattern formedon a transparent conductor sheet.

FIGS. 37A-37B show the etching rate by using a acid-etching solutionaccording to one embodiment.

FIGS. 38A and 38B illustrate a patterning and etching process accordingto one embodiment.

FIGS. 39A and 39B illustrate coating commercial color filters withnanowire-based transparent conductor film according to one embodiment.

FIGS. 40A-40B show an embodiment of patterning a transparent conductorlayer without affecting its optical property.

FIG. 41 shows another embodiment of patterning by etching.

FIGS. 42A-42B show a further embodiment of etching a transparentconductor without altering its optical properties.

FIG. 43 shows two schemes involving chemicals that can form a barrierlayer on a metal surface.

DETAILED DESCRIPTION

Certain embodiments are directed to a transparent conductor based on aconductive layer of nanostructures (e.g., nanowires). In variousembodiments, the conductive layer includes a sparse network of metalnanowires. In addition, the conductive layer is transparent, flexibleand can include at least one surface that is conductive. It can becoated or laminated on a variety of substrates, including flexible andrigid substrates. The conductive layer can also form part of a compositestructure including a matrix material or overcoat and the nanowires. Thematrix material can typically impart certain chemical, mechanical andoptical properties to the composite structure. Other embodimentsdescribe methods of fabricating and patterning the conductive layer.

Conductive Nanostructures

As used herein, “conductive nanostructures” or “nanostructures”generally refer to electrically conductive nano-sized structures, atleast one dimension of which (i.e., width or diameter) is less than 500nm, more typically, less than 100 nm or 50 nm. In various embodiments,the width or diameter of the nanostructures are in the range of 10 to 40nm, 20 to 40 nm, 5 to 20 nm, 10 to 30 nm, 40 to 60 nm, 50 to 70 nm.

The nanostructures can be of any shape or geometry. One way for definingthe geometry of a given nanostructure is by its “aspect ratio,” whichrefers to the ratio of the length and the width (or diameter) of thenanostructure. In certain embodiments, the nanostructures areisotropically shaped (i.e., aspect ratio=1). Typical isotropic orsubstantially isotropic nanostructures include nanoparticles. Inpreferred embodiments, the nanostructures are anisotropically shaped(i.e. aspect ratio≠1). The anisotropic nanostructure typically has alongitudinal axis along its length. Exemplary anisotropic nanostructuresinclude nanowires (solid nanostructures having aspect ratio of at least10, and more typically, at least 50), nanorod (solid nanostructureshaving aspect ratio of less than 10) and nanotubes (hollownanostructures).

Lengthwise, anisotropic nanostructures (e.g., nanowires) are more than500 nm, or more than 1 μm, or more than 10 μm in length. In variousembodiments, the lengths of the nanostructures are in the range of 5 to30 μm, or in the range of 15 to 50 μm, 25 to 75 μm, 30 to 60 μm, 40 to80 μm, or 50 to 100 μm.

The nanostructures can be of any conductive material. More typically,the nanostructures are formed of a metallic material, includingelemental metal (e.g., transition metals) or a metal compound (e.g.,metal oxide). The metallic material can also be a bimetallic material ora metal alloy, which comprises two or more types of metal. Suitablemetals include, but are not limited to, silver, gold, copper, nickel,gold-plated silver, platinum and palladium. It should be noted thatalthough the present disclosure describes primarily nanowires (e.g.,silver nanowires), any nanostructures within the above definition can beequally employed.

FIG. 1 illustrates a nanowire 2 having an aspect ratio equal to thelength L₁ divided by the diameter d₁. Suitable nanowires typically haveaspect ratios in the range of 10 to 100,000. Larger aspect ratios can befavored for obtaining a transparent conductor layer since they mayenable more efficient conductive networks to be formed while permittinglower overall density of wires for a high transparency. In other words,when conductive nanowires with high aspect ratios are used, the densityof the nanowires that achieves a conductive network can be low enoughthat the conductive network is substantially transparent.

One method to define the transparency of a layer to light is by itsabsorption coefficient. The illumination of light passing through alayer can be defined as:

I=I_(o)e^(−ax)

in which I_(o) is the incoming light on a first side of the layer, I isthe illumination level that is present on a second side of the layer,and e^(−ax) is the transparency factor. In the transparency factor, a isthe absorption coefficient and x is the thickness of the layer. A layerhaving a transparency factor near 1, but less than 1 can be consideredto be substantially transparent.

FIGS. 2-5 illustrate some of the optical and electrical characteristicsof the conductive nanowires.

FIG. 2 shows a theoretical model of the light absorption of silvernanoellipsoids at various wavelengths of light. Depending on widths andlengths, silver nanoellipsoids exhibit a high extinction coefficient toa narrow band of light in the wavelengths between 400 and 440 nanometersand to wavelengths of light above 700 nm. However, they aresubstantially transparent between about 440 to about 700 nm, which fallsin the visible range.

FIG. 3 shows the absorption spectrum of a layer of silver nanowiresdeposited on a polyethylene terephthalate (PET) substrate. As shown bythe absorption profile, the silver nanowire layer on PET substrate issubstantially transparent between about 440 nm to 700 nm, agreeing withthe results of the theoretical model shown in FIG. 2.

FIGS. 4 and 5 show the results of theoretical modeling of theresistivity of metal nanowires based on their diameters. For a largerdiameter of nanowire, the resistivity decreases substantially althoughit will absorb more light.

As can be seen in FIG. 4, the effects on resistivity based on the grainboundary and surface scattering are high at diameters of less than 10nm. As the diameter increases, these effects are drastically reduced.The overall resistivity is therefore reduced greatly for diameter thatincreases from 10 nm to over 100 nm (see, also FIG. 5). This improvementin electrical properties must be balanced, however, against thedecreased transparency for applications requiring a transparentconductor.

FIG. 6 shows a single Ag nanowire 4 that extends between two otherelectrical terminals 6 a and 6 b, to provide an electrically conductivepath from terminal 6 a to terminal 6 b. The term “terminal” includescontact pads, conduction nodes and any other starting and ending pointsthat may be electrically connected. The aspect ratio, size, shape andthe distribution of the physical parameters of the nanowires areselected to provide the desired optical and electrical properties. Thenumber of such wires that will provide a given density of Ag nanowiresis selected to provide acceptable electrical conduction properties forcoupling terminal 6 a to terminal 6 b. For example, hundreds of Agnanowires 4 can extend from terminal 6 a to 6 b to provide a lowresistance electrical conduction path, and the concentration, aspectratio, size and shape can be selected to provide a substantiallytransparent conductor. Therefore, transparent, electrical conduction isprovided from terminal 6 a to terminal 6 b using a plurality of Agnanowires.

As can be appreciated, the distance from terminal 6 a to terminal 6 bmay be such that the desired optical properties are not obtained with asingle nanowire. A plurality of many nanowires may need to be linked toeach other at various points to provide a conductive path from terminal6 a to terminal 6 b. According to the invention, the nanowires areselected based on the desired optical properties. Then, the number ofnanowires that provides the desired conduction path and overallresistance on that path are selected to achieve acceptable electricalproperties for an electrical conduction layer from terminal 6 a toterminal 6 b.

The electrical conductivity of the transparent layer is mainlycontrolled by a) the conductivity of a single nanowire, b) the number ofnanowires between the terminals, and c) the connectivity between thenanowires. Below a certain nanowire concentration (also referred to asthe percolation threshold, or electrical percolation level), theconductivity between the terminals is zero, i.e. there is no continuouscurrent path provided because the nanowires are spaced too far apart.Above this concentration, there is at least one current path available.As more current paths are provided, the overall resistance of the layerwill decrease.

Conductive nanowires include metal nanowires and other conductiveparticles having high aspect ratios (e.g., higher than 10). Examples ofnon-metallic nanowires include, but are not limited to, carbon nanotubes(CNTs), metal oxide nanowires, conductive polymer fibers and the like.

As used herein, “metal nanowire” refers to a metallic wire comprisingelement metal, metal alloys or metal compounds (including metal oxides).At least one cross-sectional dimension of the metal nanowire is lessthan 500 nm, and less than 200 nm, and more preferably less than 100 nm.As noted above, the metal nanowire has an aspect ratio (length:diameter)of greater than 10, preferably greater than 50, and more preferablygreater than 100. Suitable metal nanowires can be based on any metal,including without limitation, silver, gold, copper, nickel, andgold-plated silver.

The metal nanowires can be prepared by known methods in the art. Inparticular, silver nanowires can be synthesized through solution-phasereduction of a silver salt (e.g., silver nitrate) in the presence of apolyol (e.g., ethylene glycol) and poly(vinyl pyrrolidone). Large-scaleproduction of silver nanowires of uniform size can be prepared accordingto the methods described in, e.g., Xia, Y. et al., Chem. Mater. (2002),14, 4736-4745, and Xia, Y. et al., Nanoletters (2003) 3(7), 955-960.

Alternatively, the metal nanowires can be prepared using biologicaltemplates (or biological scaffolds) that can be mineralized. Forexample, biological materials such as viruses and phages can function astemplates to create metal nanowires. In certain embodiments, thebiological templates can be engineered to exhibit selective affinity fora particular type of material, such as a metal or a metal oxide. Moredetailed description of biofabrication of nanowires can be found in,e.g., Mao, C. B. et al., “Virus-Based Toolkit for the Directed Synthesisof Magnetic and Semiconducting Nanowires,” (2004) Science, 303, 213-217.Mao, C. B. et al., “Viral Assembly of Oriented Quantum Dot Nanowires,”(2003) PNAS, vol. 100, no. 12, 6946-6951; Mao, C. B. et al., “ViralAssembly of Oriented Quantum Dot Nanowires,” (2003) PNAS, 100(12),6946-6951, U.S. application Ser. No. 10/976,179 and U.S. ProvisionalApplication Ser. No. 60/680,491, which references are incorporatedherein in their entireties.

More specifically, a conductive material or a conductor (e.g., a metalnanowire) can directly bind to a biological template based on anaffinity between the conductive material and certain binding sites(e.g., peptide sequences) on the biological template.

In other embodiments, a conductive material can be created by anucleation process, during which a precursor is converted to conductiveparticles that bind to the biological templates, the conductiveparticles being capable of further growing into a continuous conductivelayer. This process is also referred to as “mineralization” or“plating”. For example, a metal precursor (e.g., a metal salt) can beconverted to an elemental metal in the presence of a reducing agent. Theresulting elemental metal binds to the biological templates and growsinto a continuous metallic layer.

In other embodiments, a seed material layer is initially nucleated ontothe biological material. Thereafter, a metal precursor can be convertedinto metal and plated on the seed material layer. The seed material canbe selected, for example, based on a material that causes the nucleationand growth of a metal out of a solution containing a corresponding metalprecursor. To illustrate, a seed material layer containing palladium cancause the mineralization of Cu or Au. As one specific example, forcreating a Cu conductor, acceptable seed materials may containpalladium, a palladium based molecule, Au or an Au based molecule. Foran oxide conductor, a zinc oxide may be used as a nucleation material.Examples of the seed material include Ni, Cu, Pd, Co, Pt, Ru, Ag, Coalloys or Ni alloys. Metals, metal alloys and metal oxides that can beplated include, without limitation, Cu, Au, Ag, Ni, Pd, Co, Pt, Ru, W,Cr, Mo, Ag, Co alloys (e.g., CoPt), Ni alloys, Fe alloys (e.g., FePt) orTiO₂, CO₃O₄, Cu₂O, HfO₂, ZnO, vanadium oxides, indium oxide, aluminumoxide, indium tin oxide, nickel oxide, copper oxide, tin oxide, tantalumoxide, niobium oxide, vanadium oxide or zirconium oxide.

Any of a number of different biological materials can be used to providethe templates for creating the metal nanowires, including proteins,peptides, phages, bacteria, viruses, and the like. The techniques forselecting, forming and engineering a biological material that willcouple to a desired metal or conductive material are described in U.S.application Ser. Nos. 10/155,883 and 10/158,596; both applications arein the name of Cambrios Technologies Corporation and are incorporatedherein by reference.

As noted above, biological templates such as protein, a peptide, orother biological material can be engineered to have affinity sites for aselected seed material or a selected conductive material. Proteins orpeptides with affinity to a specific material can be identified througha protein discovery process such as phage display, yeast display, cellsurface display or others. For example in the case of phage display,libraries of phages (e.g., M13 phages) can be created by inserting awide variety of different sequences of peptides into a population of thephage. A protein having high affinity for a specific target molecule canbe isolated and its peptide structure can be identified.

In particular, the genetic sequences of the biological molecules can becontrolled to provide a number of copies of particular peptide sequencesin certain types of phage particles. For example, about 3000 copies ofP8 proteins can be arranged in an ordered array along the length of M13phage particles. The P8 proteins can be modified to include a specificpeptide sequence that can nucleate the formation of or bind a conductivematerial, thereby providing conductive nanowires of high conductivity.Advantageously, this technique allows for the ability to control thegeometry and crystalline structure of the nanowires through the use ofbiological template molecules, e.g., proteins having specificallydesigned or controlled peptide sequences. To that end, peptides orproteins with binding affinity for silver, gold or palladium have beenidentified which can be incorporated into a phage structure to createnanowires with dimensions based on those of the phage particles.

Biological materials other than phages can be used as templates for theformation of conductive nanowires. For example, filamentous proteinswhich self-assemble into long strands of tens of microns in length canbe used as an alternative template (see, FIG. 7). Advantageously, such atemplate protein can be synthesized to have a much larger aspect ratiothan phage, which leads to lower percolative threshold concentrations ofthe conductive nanowires. Additionally, proteins are easier tosynthesize in large volume than phage particles. Large scale manufactureof proteins, such as enzymes used as detergent additives, is welldeveloped.

FIG. 8 shows a schematic version of a protein scaffold 8 having a numberof binding sites 8 a coupled with conductive particles 8 b. The bindingsites are selected to have an affinity for the conductive particles,such as Au, Ag, Cu and Ni. Alternatively, the binding sites 8 a have anaffinity for a seed material layer (e.g., Pd and Au) that can furthernucleate the conductive particles, such as Cu and the like. The proteinscaffold 8 can also be engineered to have a plurality of binding sites 8a with such affinity. It is preferred to have them spaced at frequentand regular intervals along their length to increase the conductivity ofthe final conductive layer.

The length of a biological material, such as a protein, as well as itsdiameter is easily engineered using known techniques. It is engineeredto have the correct dimensions for the optical properties. Once thesize, shape and aspect ratio have been selected, the biological materialcan be exposed to conductive material 8 b, such as a metal, or aprecursor of the metal.

FIG. 9 illustrates a further embodiment of fabricating conductivenanowires using biological templates. The protein scaffold 8 can befurther engineered to include binding partners such as associatepeptides 9 a and 9 b at respective ends. Binding partners can couplewith each other through any type of associative interaction, including,for example, ionic interaction, covalent bonding, hydrogen bonding,hydrophobic interaction, and the like. The interaction between theassociate peptides 9 a and 9 b encourage the self-assembly of theconductive nanowires into 2-D interconnected mesh networks, as shown inthe final sequence in FIG. 8. The association peptides and theirlocations may be of the type to encourage the forming of meshes, end toend connection, cross-connections, and other desired shapes for theconductive layer. In the example shown in FIG. 8, the conductivematerial 8 b has already bound to the protein scaffold 8 before theprotein scaffolds form a network. It should be understood, that proteinscaffold 8 can also form a network prior to the binding of theconductive material.

Thus, the use of biological template having associate peptides or otherbinding partners allows for the formation of a conductive layer ofhighly connected network than would be possible with random nanowires.The particular network of the biological templates can therefore beselected to achieve a desired degree of order in the conductive layer.

Template-based synthesis is particularly suited for fabricatingnanowires of particular dimensions, morphologies and compositions.Further advantages of biologically based manufacturing of nano-materialsinclude: solution processing that can be modified for high throughput,ambient temperature deposition, superior conformality and production ofconductive layers.

Conductive Layer and Substrate

As an illustrative example, FIG. 10A shows a transparent conductor 10comprising a conductive layer 12 coated on a substrate 14. Theconductive layer 12 comprises a plurality of metal nanowires 16. Themetal nanowires form a conductive network.

FIG. 10B shows another example of a transparent conductor 10′, in whicha conductive layer 12′ is formed on the substrate 14. The conductivelayer 12′ includes a plurality of metal nanowires 16 embedded in amatrix 18.

“Matrix” refers to a solid-state material into which the metal nanowiresare dispersed or embedded. Portions of the nanowires may protrude fromthe matrix material to enable access to the conductive network. Thematrix is a host for the metal nanowires and provides a physical form ofthe conductive layer. The matrix protects the metal nanowires fromadverse environmental factors, such as corrosion and abrasion. Inparticular, the matrix significantly lowers the permeability ofcorrosive elements in the environment, such as moisture, trace amount ofacids, oxygen, sulfur and the like.

In addition, the matrix offers favorable physical and mechanicalproperties to the conductive layer. For example, it can provide adhesionto the substrate. Furthermore, unlike metal oxide films, polymeric ororganic matrices embedded with metal nanowires are robust and flexible.As will be discussed in more detail herein, flexible matrices make itpossible to fabricate transparent conductors in a low-cost and highthroughput process.

Moreover, the optical properties of the conductive layer can be tailoredby selecting an appropriate matrix material. For example, reflectionloss and unwanted glare can be effectively reduced by using a matrix ofa desirable refractive index, composition and thickness.

Typically, the matrix is an optically clear material. A material isconsidered “optically clear” or “optically transparent”, if the lighttransmission of the material is at least 80% in the visible region (400nm-700 nm). Unless specified otherwise, all the layers (including thesubstrate and the nanowire network layer) in a transparent conductordescribed herein are preferably optically clear. The optical clarity ofthe matrix is typically determined by a multitude of factors, includingwithout limitation: the refractive index (RI), thickness, consistency ofRI throughout the thickness, surface (including interface) reflection,and haze (a scattering loss caused by surface roughness and/or embeddedparticles).

In certain embodiments, the matrix is about 10 nm to 5 μm thick, about20 nm to 1 μm thick, or about 50 nm to 200 nm thick. In otherembodiments, the matrix has a refractive index of about 1.3 to 2.5, orabout 1.35 to 1.8.

In certain embodiments, the matrix is a polymer, which is also referredto as a polymeric matrix. Optically clear polymers are known in the art.Examples of suitable polymeric matrices include, but are not limited to:polyacrylics such as polymethacrylates (e.g., poly(methylmethacrylate)), polyacrylates and polyacrylonitriles, polyvinylalcohols, polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonates), polymers with a high degree ofaromaticity such as phenolics or cresol-formaldehyde (Novolacs®),polystyrenes, polyvinyltoluene, polyvinylxylene, polyimides, polyamides,polyamideimides, polyetherimides, polysulfides, polysulfones,polyphenylenes, and polyphenyl ethers, polyurethane (PU), epoxy,polyolefins (e.g. polypropylene, polymethylpentene, and cyclic olefins),acrylonitrile-butadiene-styrene copolymer (ABS), cellulosics, siliconesand other silicon-containing polymers (e.g. polysilsesquioxanes andpolysilanes), polyvinylchloride (PVC), polyacetates, polynorbornenes,synthetic rubbers (e.g., EPR, SBR, EPDM), and fluoropolymers (e.g.,polyvinylidene fluoride, polytetrafluoroethylene (TFE) orpolyhexafluoropropylene), copolymers of fluoro-olefin and hydrocarbonolefin (e.g., Lumiflon®), and amorphous fluorocarbon polymers orcopolymers (e.g., CYTOP® by Asahi Glass Co., or Teflon® AF by Du Pont).

In other embodiments, the polymeric matrix described herein comprisespartially polymerized or partially cured polymer. Compared to a fullypolymerized or fully cured matrix, a partially cured matrix has lesserdegree of cross-linking and/or polymerization and lower molecularweight. Thus, the partially polymerized matrix can be etched undercertain conditions and patterning is possible using conventionalphotolithography. Under a proper polymerization condition, the partiallycured matrix may be further cured whereby further cross-linking andpolymerization are carried out to provide a matrix of higher molecularweight than that of a partially cured matrix. The partially cured matrixcan be etched, followed by a further curing step, to provide a patternedand fully-cured transparent conductor film. Examples of suitablepartially cured polymers include, but are not limited to partially curedacrylate, silicone-epoxy, siloxane, novolac, epoxy, urethane,silsesquioxane or polyimide.

One skilled in the art would recognize that the degree of polymerizationmay impact the etching condition (solution) under which the partiallypolymerized matrix and/or nanowires can dissolve. Typically, the higherthe degree of polymerization, the more difficult it is to etch thematrix.

Preferably, the partially cured matrix has an acceptable degree ofphysical integrity to protect the nanowires within. This is desirablebecause an end-user may carry out his own patterning and the subsequentcuring to obtain a final transparent conductor film.

In further embodiments, the matrix is an inorganic material. Forexample, a sol-gel matrix based on silica, mullite, alumina, SiC,MgO—Al₂O₃—SiO₂, Al2O₃—SiO₂, MgO—Al₂O₃—SiO₂—Li₂O or a mixture thereof canbe used.

In certain embodiments, the matrix itself is conductive. For example,the matrix can be a conductive polymer. Conductive polymers are wellknown in the art, including without limitation:poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines, polythiophenes,and polydiacetylenes.

“Conductive layer”, or “conductive film”, refers to a network layer ofmetal nanowires that provide the conductive media of the transparentconductor. When a matrix is present, the combination of the networklayer of metal nanowires and the matrix is also referred to as a“conductive layer”. Since conductivity is achieved by electrical chargepercolating from one metal nanowire to another, sufficient metalnanowires must be present in the conductive layer to reach an electricalpercolation threshold and become conductive. The surface conductivity ofthe conductive layer is inversely proportional to its surfaceresistivity, sometimes referred to as sheet resistance, which can bemeasured by known methods in the art.

Likewise, when a matrix is present, the matrix must be filled withsufficient metal nanowires to become conductive. As used herein,“threshold loading level” refers to a percentage of the metal nanowiresby weight after loading of the conductive layer at which the conductivelayer has a surface resistivity of no more than about 10⁶ ohm/square (orΩ/□). More typically, the surface resistivity is no more than 10⁵ Ω/□,no more than 10⁴ Ω/□, no more than 1,000 Ω/□, no more than 500 Ω/□, orno more than 100 Ω/□. The threshold loading level depends on factorssuch as the aspect ratio, the degree of alignment, degree ofagglomeration and the resistivity of the metal nanowires.

As is understood by one skilled in the art, the mechanical and opticalproperties of the matrix are likely to be altered or compromised by ahigh loading of any particles therein. Advantageously, the high aspectratios of the metal nanowires allow for the formation of a conductivenetwork through the matrix at a threshold surface loading levelpreferably of about 0.05 μg/cm² to about 10 μg/cm², more preferably fromabout 0.1 μg/cm² to about 5 μg/cm² and more preferably from about 0.8μg/cm² to about 3 μg/cm² for silver nanowires. These surface loadinglevels do not affect the mechanical or optical properties of the matrix.These values depend strongly on the dimensions and spatial dispersion ofthe nanowires. Advantageously, transparent conductors of tunableelectrical conductivity (or surface resistivity) and opticaltransparency can be provided by adjusting the loading levels of themetal nanowires.

In certain embodiments, the conductive layer spans the entire thicknessof the matrix, as shown in FIG. 10B. Advantageously, a certain portionof the metal nanowires is exposed on a surface 19 of the matrix due tothe surface tension of the matrix material (e.g., polymers). Thisfeature is particularly useful for touch screen applications. Inparticular, a transparent conductor can display surface conductivity onat least one surface thereof. FIG. 100 illustrates how it is believedthe network of metal nanowires embedded in a matrix achieves surfaceconductivity. As shown, while some nanowires, such as nanowire 16 a, maybe entirely ‘submerged’ in the matrix 18, ends of other nanowires, suchas end 16 b, protrude above the surface 19 of the matrix 18. Also, aportion of a middle section of nanowires, such as middle section 16 c,may protrude above the surface 19 of the matrix 18. If enough nanowireends 16 b and middle sections 16 c protrude above the matrix 18, thesurface of the transparent conductor becomes conductive. FIG. 10D is ascanning electron micrograph of the surface of one embodiment of atransparent conductor showing a contour of ends and middle sections ofnanowires protruding above a matrix in a transparent conductor.

In other embodiments, the conductive layer is formed by the metalnanowires embedded in a portion of the matrix, as shown in FIG. 10E. Theconductive layer 12″ occupies only a portion of the matrix 18 and arecompletely “submerged” in the matrix 18.

In addition, the nanowire-filled conductive layer may be overlaid withone or more dielectric coatings (e.g., an overcoat or an anti-glarefilm) which protect or enhance the performance of the conductive layer.In these circumstances, the conductive layer may not be surfaceconductive, but is characterized with in-plane conductivity.

In certain embodiments, surface conductivity in the overcoat can beestablished by incorporating a plurality of nano-sized conductiveparticles in the overcoat. As shown in FIG. 10F, a nanowire-basedconductive layer 10 is deposited on substrate 14. The conductive layer10 comprises nanowires 16 which reach the percolation threshold andestablish in-plane conductivity. An overcoat 17 is formed on theconductive layer 10. A plurality of conductive particles 17 a isembedded in the overcoat 17. Advantageously, the loading level of thenano-sized conductive particles in the overcoat does not need to reachthe percolation threshold to exhibit surface conductivity. Theconductive layer remains as the current-carrying medium, in which thenanowires have reached electrical percolation level. The conductiveparticles in the overcoat provide for surface conductivity as a resultof their contacts with the underlying nanowires through the thickness ofthe overcoat.

Thus, one embodiment provides a multi-layer structure comprising: asubstrate; a conductive layer formed on the substrate, wherein theconductive layer comprises a first plurality of metallic nanowires, thefirst plurality of metallic nanowires reaching an electrical percolationlevel; and an overcoat formed on the conductive layer, the overcoatincorporating a second plurality of conductive particles, the secondplurality of conductive particles being below the electrical percolationlevel.

As used herein, nano-sized conductive particles refer to conductiveparticles having at least one dimension that is no more than 500 nm,more typically, no more than 200 nm. Examples of suitable conductiveparticles include, but are not limited to, ITO, ZnO, doped ZnO, metallicnanowires (including those described herein), metallic nanotubes, carbonnanotubes (CNT) and the like.

“Substrate”, or “substrate of choice”, refers to a material onto whichthe conductive layer is coated or laminated. The substrate can be rigidor flexible. The substrate can be clear or opaque. The term “substrateof choice” is typically used in connection with a lamination process, aswill be discussed herein. Suitable rigid substrates include, forexample, glass, polycarbonates, acrylics, and the like. Suitableflexible substrates include, but are not limited to: polyesters (e.g.,polyethylene terephthalate (PET), polyester naphthalate, andpolycarbonate), polyolefins (e.g., linear, branched, and cyclicpolyolefins), polyvinyls (e.g., polyvinyl chloride, polyvinylidenechloride, polyvinyl acetals, polystyrene, polyacrylates, and the like),cellulose ester bases (e.g., cellulose triacetate, cellulose acetate),polysulphones such as polyethersulphone, polyimides, silicones and otherconventional polymeric films. Additional examples of suitable substratescan be found in, e.g., U.S. Pat. No. 6,975,067.

Typically, the optical transparence or clarity of the conductive layercan be quantitatively defined by parameters including light transmissionand haze. “Light transmission” (or “light transmissivity”) refers to thepercentage of an incident light transmitted through a medium. In variousembodiments, the light transmission of the conductive layer is at least80% and can be as high as 98%. For a transparent conductor in which theconductive layer is deposited or laminated on a substrate, the lighttransmission of the overall structure may be slightly diminished.Performance-enhancing layers, such as an adhesive layer, anti-reflectivelayer, anti-glare layer, may further contribute to reducing the overalllight transmission of the transparent conductor. In various embodiments,the light transmission (“T %) of the transparent conductors can be atleast 50%, at least 60%, at least 70%, or at least 80% and may be ashigh as at least 91% to 92%, or at least 95%.

Haze (H %) is an index of light diffusion. It refers to the percentageof the quantity of light separated from the incident light and scatteredduring transmission. Unlike light transmission, which is largely aproperty of the medium, haze is often a production concern and istypically caused by surface roughness and embedded particles orcompositional heterogeneities in the medium. Typically, haze of aconductive film can be significantly impacted by the diameters of thenanostructures. Nanostructures of larger diameters (e.g., thickernanowires) are typically associated with a higher haze. In variousembodiments, the haze of the transparent conductor is no more than 10%,no more than 8%, or no more than 5% and may be as low as no more than2%, no more than 1%, or no more than 0.5%, or no more than 0.25%.

Performance-Enhancing Layers

As noted above, the conductive layers have superior physical andmechanical characteristics due to the matrix. These characteristics canbe further enhanced by introducing additional layers in the transparentconductor structure. Thus, in other embodiments, a multi-layertransparent conductor is described, which comprises one or more layerssuch as anti-reflective layers, anti-glare layers, adhesive layers,barrier layers, and hard coats.

As an illustrative example, FIG. 11 shows a multi-layer transparentconductor 20 comprising a conductive layer 12 and a substrate 14, asdescribed above. The multi-layer transparent conductor 20 furthercomprises a first layer 22 positioned over the conductive layer 12, asecond layer 24 positioned between the conductive layer 12 and thesubstrate 14, and a third layer 26 positioned below the substrate 14.Unless stated otherwise, each of the layers 22, 24 and 26 can be one ormore anti-reflective layers, anti-glare layers, adhesive layers, barrierlayers, hard coats, and protective films.

The layers 22, 24 and 26 serve various functions, such as enhancing theoverall optical performance and improving the mechanical properties ofthe transparent conductor. These additional layers, also referred to as“performance-enhancing layers”, can be one or more anti-reflectivelayers, anti-glare layers, adhesive layers, barrier layers, and hardcoats. In certain embodiments, one performance-enhancing layer providesmultiple benefits. For example, an anti-reflective layer can alsofunction as a hard coat and/or a barrier layer. In addition to theirspecific properties, the performance-enhancing layers are opticallyclear, as defined herein.

In one embodiment, layer 22 is an anti-reflective layer, layer 24 is anadhesive layer, and layer 26 is a hard coat.

In another embodiment, layer 22 is a hard coat, layer 24 is a barrierlayer, and layer 26 is an anti-reflective layer.

In yet another embodiment, layer 22 is a combination of ananti-reflective layer, anti-glare, a barrier layer and a hard coat,layer 24 is an adhesive layer, and layer 26 is an anti-reflective layer.

“Anti-reflective layer” refers to a layer that can reduce reflectionloss at a reflective surface of the transparent conductor. Theanti-reflective layer can therefore be positioned on the outer surfacesof the transparent conductor, or as an interface between layers.Materials suitable as anti-reflective layers are well known in the art,including without limitation: fluoropolymers, fluoropolymer blends orcopolymers, see, e.g., U.S. Pat. Nos. 5,198,267, 5,225,244, and7,033,729.

In other embodiments, reflection loss can be effectively reduced bycontrolling the thickness of the anti-reflective layer. For example,with reference to FIG. 11, the thickness of layer 22 can be controlledsuch that the light reflection of surface 28 and surface 30 cancel eachother out. Thus, in various embodiments, the anti-reflective layer isabout 100 nm thick or 200 nm thick.

Reflection loss can also be reduced by the appropriate use of texturedsurfaces, see, e.g. U.S. Pat. No. 5,820,957 and literature on AutoflexMARAG™ and Motheye™ products from MacDiarmid Autotype.

“Anti-glare layer” refers to a layer that reduces unwanted reflection atan outer surface of the transparent conductor by providing fineroughness on the surface to scatter the reflection. Suitable anti-glarematerials are well known in the art, including without limitation,siloxanes, polystyrene/PMMA blend, lacquer (e.g., butylacetate/nitrocellulose/wax/alkyd resin), polythiophenes, polypyrroles,polyurethane, nitrocellulose, and acrylates, all of which may comprise alight diffusing material such as colloidal or fumed silica. See, e.g.,U.S. Pat. Nos. 6,939,576, 5,750,054, 5,456,747, 5,415,815, and5,292,784. Blends and copolymers of these materials can have microscalecompositional heterogeneities, which can also exhibit light diffusionbehavior to reduce glare.

“Hard coat”, or “anti-abrasion layer” refers to a coating that providesadditional surface protection against scratches and abrasion. Examplesof suitable hard coats include synthetic polymers such as polyacrylics,epoxy, polyurethanes, polysilanes, silicones, poly(silico-acrylic) andso on. Typically, the hard coat also comprises colloidal silica. (See,e.g., U.S. Pat. Nos. 5,958,514, 7,014,918, 6,825,239, and referencescited therein.) The thickness of the hard coat is typically from about 1to 50 μm. The degree of hardness can be evaluated by known methods inthe art, such as by scratching the coating with a steel wool #000reciprocating 50 times within a distance of 2 cm at 2 reciprocations/secunder load of 300 g/cm² (see, e.g., U.S. Pat. No. 6,905,756). The hardcoat may be further exposed to an anti-glare process or ananti-reflection treatment by known methods in the art.

“Adhesive layer” refers to any optically clear material that bonds twoadjacent layers (e.g., conductive layer and substrate) together withoutaffecting the physical, electrical or optical properties of eitherlayer. Optically clear adhesive material are well known in the art,including without limitation: acrylic resins, chlorinated olefin resins,resins of vinyl chloride-vinyl acetate copolymer, maleic acid resins,chlorinated rubber resins, cyclorubber resins, polyamide resins,cumarone indene resins, resins of ethylene-vinyl acetate copolymer,polyester resins, urethane resins, styrene resins, polysiloxanes and thelike.

“Barrier layer” refers to a layer that reduces or prevents gas or fluidpermeation into the transparent conductor. It has been shown thatcorroded metal nanowires can cause a significant decrease in theelectrical conductivity as well as the light transmission of theconductive layer. The barrier layer can effectively inhibit atmosphericcorrosive gas from entering the conductive layer and contacting themetal nanowires in the matrix. The barrier layers are well known in theart, including without limitation: see, e.g. U.S. Patent Application No.2004/0253463, U.S. Pat. Nos. 5,560,998 and 4,927,689, EP Patent No.132,565, and JP Patent No. 57,061,025. Moreover, any of theanti-reflective layer, anti-glare layer and the hard coat can also actas a barrier layer.

In certain embodiments, the multi-layer transparent conductor mayfurther comprise a protective film above the conductive layer (e.g.,layer 22). The protective film is typically flexible and can be made ofthe same material as the flexible substrate. Examples of protective filminclude, but are not limited to: polyester, polyethylene terephthalate(PET), polybutylene terephthalate, polymethyl methacrylate (PMMA),acrylic resin, polycarbonate (PC), polystyrene, triacetate (TAO),polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride,polyethylene, ethylene-vinyl acetate copolymers, polyvinyl butyral,metal ion-crosslinked ethylene-methacrylic acid copolymers,polyurethane, cellophane, polyolefins or the like; particularlypreferable is PET, PC, PMMA, or TAO because of their high strength.

Corrosion Inhibitors

In other embodiments, the transparent conductor may comprise a corrosioninhibitor, in addition to, or in lieu of the barrier layer as describedabove. Different corrosion inhibitors may provide protection to themetal nanowires based on different mechanisms. It should be understoodthat an appropriate selection of the corrosion inhibitor(s) can offer arange of protections to the metal nanowires against adverseenvironmental impacts, including both oxidation and sulfidation.

According to one mechanism, the corrosion inhibitor binds readily to themetal nanowires, forming a protective film on a metal surface. They arealso referred to as barrier-forming corrosion inhibitors.

In one embodiment, the barrier-forming corrosion inhibitor includescertain nitrogen-containing and sulfur-containing organic compounds,such as aromatic triazoles, imidazoles and thiazoles. These compoundshave been demonstrated to form stable complexes on a metal surface toprovide a barrier between the metal and its environment. For example,benzotriazole (BTA) is a common organic corrosion inhibitor for copperor copper alloys (Scheme 1 of FIG. 43). Alkyl substitutedbenzotriazoles, such as tolytriazole and butyl benzyl triazole, can alsobe used. (See, e.g., U.S. Pat. No. 5,270,364.) Additional suitableexamples of corrosion inhibitors include, but are not limited to:2-aminopyrimidine, 5,6-dimethylbenzimidazole,2-amino-5-mercapto-1,3,4-thiadiazole, 2-mercaptopyrimidine,2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and2-mercaptobenzimidazole.

Another class of barrier-forming corrosion inhibitors includesbiomolecules that show a particular affinity to the metal surface. Theseinclude small biomolecules, e.g. cysteine, and synthetic peptides andprotein scaffolds with fused peptide sequences with affinity for metals,e.g. EEEE; see, e.g. U.S. application Ser. Nos. 10/654,623, 10/665,721,10/965,227, 10/976,179, and 11/280,986, U.S. Provisional ApplicationSer. Nos. 60/680,491, 60/707,675 and 60/680,491.

Other barrier-forming corrosion inhibitors include dithiothiadiazole,alkyl dithiothiadiazoles and alkylthiols, alkyl being a saturated C₆-C₂₄straight hydrocarbon chain. This type of corrosion inhibitor canself-assemble on a metal surface to form a monolayer (Scheme 2 of FIG.43), thereby protecting the metal surface from corroding.

In a particular embodiment, the transparent conductor can comprise areservoir containing a corrosion inhibitor, providing a continuoussupply of the corrosion inhibitor in the vapor phase. The corrosioninhibitors suitable for such sustained delivery include “vapor phaseinhibitors” (VPI). VPIs are typically volatile solid materials thatsublime and form a monolayer on the surfaces of the metal nanowires.Advantageously, VPIs can be delivered to the metal surfaces andreplenished in a sustained manner for long-lasting protection. SuitableVPIs include barrier-forming inhibitors such as triazoles,dithiothiadiazole, alkyl dithiothiadiazoles and alkylthiols, asdescribed herein.

FIG. 12 illustrates such a transparent conductor structure suitable fora touch screen. More specifically, edge seals 32 and spacers 36 arepositioned between two conductive layers 12. In the space between thetwo conductive layers 12, one or more reservoirs 40 are present. Thereservoirs 40 are microscopic and are sparsely distributed such thattheir presence does not cause a reduction in the transmittance of thetransparent conductor. The reservoir contains a corrosion inhibitorwhich can be incorporated into a polymer matrix or impregnated into aporous material from which it can be sublimated into the vapor phase toform a monolayer 44 on the surface of the metal nanowires (see, inset).

According to another mechanism, a corrosion inhibitor binds more readilywith a corrosive element (e.g., H₂S) than with the metal nanowires.These corrosion inhibitors are known as “scavengers” or “getters”, whichcompete with the metal and sequester the corrosive elements. Examples ofH₂S scavengers include, but are not limited to: acrolein, glyoxal,triazine, and n-chlorosuccinimide. (See, e.g., Published U.S.Application No. 2006/0006120.)

In certain embodiments, the corrosion inhibitor (e.g., H₂S scavengers)can be dispersed in the matrix provided its presence does not adverselyaffect the optical or electrical properties of the conductive layer.

In other embodiments, the metal nanowires can be pretreated with acorrosion inhibitor before or after being deposited on the substrate.For example, the metal nanowires can be pre-coated with abarrier-forming corrosion inhibitor, e.g., BTA and dithiothiadiazole. Inaddition, the metal nanowires can also be treated with an anti-tarnishsolution. Metal anti-tarnish treatments are known in the art. Specifictreatments targeting H₂S corrosion are described in, e.g., U.S. Pat. No.4,083,945, and U.S. Published Application No. 2005/0148480.

In yet other embodiments, the metal nanowires can be alloyed or platedwith another metal less prone to corrosion by atmospheric elements. Forexample, silver nanowires can be plated with gold, which is lesssusceptible to oxidation and sulfidation.

In certain embodiments, the transparent conductors described herein canbe fabricated by various coating methods, including sheet coating andhigh throughput web coating. In other embodiments, a laminating methodcan be used. Advantageously, the fabrication processes described hereindo not require vacuum deposition, in contrast to the current fabricationof the metal oxide films. Instead, the fabrication processes can becarried out using conventional solution-processing equipment. Moreover,the fabrication processes are compatible with directly patterning thetransparent conductor.

Nanowire Deposition and Transparent Conductor Fabrication

In certain embodiments, it is thus described herein a method offabricating a transparent conductor comprising: depositing a pluralityof metal nanowires on a substrate, the metal nanowires being dispersedin a fluid; and forming a metal nanowire network layer on the substrateby allowing the fluid to dry.

The metal nanowires can be prepared as described above. The metalnanowires are typically dispersed in a liquid to facilitate thedeposition. It is understood that, as used herein, “deposition” and“coating” are used interchangeably. Any non-corrosive liquid in whichthe metal nanowires can form a stable dispersion (also called “metalnanowires dispersion”) can be used. Preferably, the metal nanowires aredispersed in water, an alcohol, a ketone, ethers, hydrocarbons or anaromatic solvent (benzene, toluene, xylene, etc.). More preferably, theliquid is volatile, having a boiling point of no more than 200° C., nomore than 150° C., or no more than 100° C.

In addition, the metal nanowire dispersion may contain additives andbinders to control viscosity, corrosion, adhesion, and nanowiredispersion. Examples of suitable additives and binders include, but arenot limited to, carboxy methyl cellulose (CMC), 2-hydroxy ethylcellulose (HEC), hydroxy propyl methyl cellulose (HPMC), methylcellulose (MC), poly vinyl alcohol (PVA), tripropylene gylcol (TPG), andxanthan gum (XG), and surfactants such as ethoxylates, alkoxylates,ethylene oxide and propylene oxide and their copolymers, sulfonates,sulfates, disulfonate salts, sulfosuccinates, phosphate esters, andfluorosurfactants (e.g., Zonyl® by DuPont).

In one example, a nanowire dispersion, or “ink” includes, by weight,from 0.0025% to 0.1% surfactant (e.g., a preferred range is from 0.0025%to 0.05% for Zonyl® FSO-100), from 0.02% to 4% viscosity modifier (e.g.,a preferred range is 0.02% to 0.5% for HPMC), from 94.5% to 99.0%solvent and from 0.05% to 1.4% metal nanowires. Representative examplesof suitable surfactants include Zonyl® FSN, Zonyl® FSO, Zonyl® FSH,Triton (x100, x114, x45), Dynol (604, 607), n-Dodecyl b-D-maltoside andNovek. Examples of suitable viscosity modifiers include hydroxypropylmethyl cellulose (HPMC), methyl cellulose, xanthan gum, polyvinylalcohol, carboxy methyl cellulose, and hydroxy ethyl cellulose. Examplesof suitable solvents include water and isopropanol.

The nanowire concentration in the dispersion can affect or determineparameters such as thickness, conductivity (including surfaceconductivity), optical transparency, and mechanical properties of thenanowire network layer. The percentage of the solvent can be adjusted toprovide a desired concentration of the nanowires in the dispersion. Inpreferred embodiments the relative ratios of the other ingredients,however, can remain the same. In particular, the ratio of the surfactantto the viscosity modifier is preferably in the range of about 80 toabout 0.01; the ratio of the viscosity modifier to the metal nanowiresis preferably in the range of about 5 to about 0.000625; and the ratioof the metal nanowires to the surfactant is preferably in the range ofabout 560 to about 5. The ratios of components of the dispersion may bemodified depending on the substrate and the method of application used.The preferred viscosity range for the nanowire dispersion is betweenabout 1 and 100 cP.

Optionally, the substrate can be pre-treated to prepare a surface tobetter receive the subsequent deposition of the nanowires. Surfacepre-treatments serve multiple functions. For example, they enable thedeposition of a uniform nanowire dispersion layer. In addition, they canimmobilize the nanowires on the substrate for subsequent processingsteps. Moreover, the pre-treatment can be carried out in conjunctionwith a patterning step to create patterned deposition of the nanowires.As will be discussed further in more detail below, pre-treatmentsinclude solvent or chemical washing, heating, deposition of anoptionally patterned intermediate layer to present an appropriatechemical or ionic state to the nanowire dispersion, as well as furthersurface treatments such as plasma treatment, UV-ozone treatment, orcorona discharge.

Following the deposition, the liquid is removed by evaporation. Theevaporation can be accelerated by heating (e.g., baking). The resultingnanowire network layer may require post-treatment to render itelectrically conductive. This post-treatment can be a process stepinvolving exposure to heat, plasma, corona discharge, UV-ozone, orpressure as described below.

In certain embodiments, it is thus described herein a method offabricating a transparent conductor comprising: depositing a pluralityof metal nanowires on a substrate, the metal nanowires being dispersedin a fluid; forming a metal nanowire network layer on the substrate byallowing the fluid to dry, coating a matrix material on the metalnanowire network layer, and curing the matrix material to form a matrix.

“Matrix material” refers to a material or a mixture of materials thatcan cure into the matrix, as defined herein. “Cure”, or “curing”, refersto a process where monomers or partial polymers (fewer than 150monomers) polymerize and/or cross-link so as to form a solid polymericmatrix. Suitable polymerization conditions are well known in the art andinclude by way of example, heating the monomer, irradiating the monomerwith visible or ultraviolet (UV) light, electron beams, and the like. Inaddition, “solidification” of a polymer/solvent system simultaneouslycaused by solvent removal is also within the meaning of “curing”.

The degree of curing can be controlled by selecting the initialconcentrations of monomers and the amount of the cross-linkers. It canbe further manipulated by adjusting curing parameters such as the timeallowed for the polymerization and the temperature under which thepolymerization takes place, and the like. In certain embodiments, thepartially cured matrix can be quenched in order to arrest the curingprocess. The degree of curing or polymerization can be monitored, forexample, by the molecular weight of the curing polymer or by the opticalabsorbance at wavelengths indicative of the reactive chemical species.

Thus, in certain embodiments, the matrix material comprises a polymer,which may be fully or partially cured. Optically clear polymers areknown in the art. Examples of suitable polymeric matrices include, butare not limited to: polyacrylates (or “acrylates”) such aspolymethacrylates, polyacrylates and polyacrylonitriles, polyvinylalcohols, polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonates), polymers with a high degree ofaromaticity such as phenolics or cresol-formaldehyde (Novolacs®),polystyrenes, polyvinyltoluene, polyvinylxylene, polyimides, polyamides,polyamideimides, polyetherimides, polysulfides, polysulfones,polyphenylenes, and polyphenyl ethers, polyurethane (PU), epoxy,silicone-epoxy, polyolefins (e.g. polypropylene, polymethylpentene, andcyclic olefins), acrylonitrile-butadiene-styrene copolymer (ABS),cellulosics, silicones and other silicon-containing polymers (e.g.polysilsesquioxanes and polysilanes), silicone-siloxane,polyvinylchloride (PVC), polyacetates, polynorbornenes, syntheticrubbers (e.g. EPR, SBR, EPDM), and fluoropolymers (e.g., polyvinylidenefluoride, polytetrafluoroethylene (TFE) or polyhexafluoropropylene),copolymers of fluoro-olefin and hydrocarbon olefin (e.g., Lumiflon®),and amorphous fluorocarbon polymers or copolymers (e.g., CYTOP® by AsahiGlass Co., or Teflon® AF by Du Pont).

In other embodiments, the matrix material comprises a prepolymer. A“prepolymer” refers to a mixture of monomers or a mixture of oligomersor partial polymers that can polymerize and/or crosslink to form thepolymeric matrix, as described herein. It is within the knowledge of oneskilled in the art to select, in view of a desirable polymeric matrix, asuitable monomer or partial polymer.

In a preferred embodiment, the prepolymer is photo-curable, i.e., theprepolymer polymerizes and/or cross-links upon exposure to irradiation.As will be described in more detail, matrices based on photo-curableprepolymers can be patterned by exposure to irradiation in selectiveregions. In other embodiments, the prepolymer is thermal-curable, whichcan be patterned by selective exposure to a heat source.

Typically, the matrix material is a liquid. The matrix material mayoptionally comprise a solvent. Any non-corrosive solvent that caneffectively solvate or disperse the matrix material can be used.Examples of suitable solvents include water, an alcohol, a ketone,tetrahydrofuran, hydrocarbons (e.g. cyclohexane) or an aromatic solvent(benzene, toluene, xylene, etc.). More preferrably, the solvent isvolatile, having a boiling point of no more than 200° C., no more than150° C., or no more than 100° C.

In certain embodiments, the matrix material may comprise a cross-linker,a polymerization initiator, stabilizers (including, for example,antioxidants, and UV stabilizers for longer product lifetime andpolymerization inhibitors for greater shelf-life), surfactants and thelike. In other embodiments, the matrix material may further comprise acorrosion inhibitor.

As noted herein, the transparent conductors can be fabricated by, forexample, sheet coating, web-coating, printing, and lamination.

(a) Sheet Coating

Sheet coating is suitable for coating a conductive layer on anysubstrate, in particular, rigid substrates.

FIGS. 13A-13B show an embodiment of the fabrication of the transparentconductor by sheet coating. A metal nanowires dispersion (not shown) canbe initially deposited to the substrate 14. A roller 100 can be rolledacross a top surface 105 of the substrate 14, leaving a metal nanowiresdispersion layer 110 on the top surface 105 (FIG. 13A). The layer 110 isallowed to dry and a metal nanowire network layer 114 is formed on thesurface 105 (FIG. 13B).

The substrate may require a pre-treatment to enable the deposition of auniform nanowire dispersion layer 110 that adheres to the substrate forsubsequent processing steps. This treatment can include solvent orchemical washing, heating, deposition of an optionally patternedintermediate layer to present an appropriate chemical or ionic state tothe nanowire dispersion, as well as further surface treatments such asplasma treatment, UV-ozone treatment, or corona discharge.

For example, an intermediate layer can be deposited on the surface ofthe substrate to immobilize the nanowires. The intermediate layerfunctionalizes and modifies the surface to facilitate the binding of thenanowires to the substrate. In certain embodiments, the intermediatelayer can be coated on the substrate prior to depositing the nanowires.In other embodiments, the intermediate layer can be co-deposited withthe nanowires.

In certain embodiments, multifunctional biomolecules such aspolypeptides can be used as the intermediate layer. Polypeptide refersto a polymeric sequence of amino acids (monomers) joined by peptide(amide) bonds. The amino acid monomers in a polypeptide can be the sameor different. Amino acids having side chain functionalities (e.g., aminoor carboxylic acid groups) are preferred. Examples of suitablepolypeptides thus include poly-L-lysine, poly-L-glutamic acid and thelike. The polypeptide can be coated on the substrate prior to thenanowire deposition. Alternatively, the polypeptide can be co-depositedon the substrate with the nanowire dispersion. Many substrates,including glass, polyester substrates (e.g., polyethylene terephthalate)exhibit affinities for polypeptides.

Advantageously, the intermediate layer can be deposited in apre-determined pattern, which enables the deposition of the nanowiresaccording to the same pattern.

Other pre-treatment methods can also be carried out in conjunction witha patterning step in order to perform patterned depositions. Forexample, plasma surface treatment can be carried out through an aperturemask having a desired pattern. The surface of the substrate thereforecomprises at least one pre-treated regions and at least one untreatedregion. Nanowires deposited on the pre-treated region adhere to thesubstrate better than they adhere to the untreated region. Accordingly,a patterned deposition can be achieved by removing the nanowires on theuntreated region by, e.g., washing.

It should be understood that the pre-treatments described above alsoapply to other methods of fabricating transparent conductors inaccordance with the description below.

The nanowire network layer formed may further require a post-treatmentto render it electrically conductive. This post-treatment can be aprocess step involving exposure to heat, plasma, corona discharge,UV-ozone, or pressure, as will be discussed in more detail below.

In some embodiments, a matrix material can be coated on the nanowirenetwork layer 114 to form a matrix material layer 116 (FIG. 13C). Asshown in FIG. 13D, the matrix material layer 116 is allowed to cure toobtain a matrix and the structures of FIGS. 10A-10E, can be obtained.

It is understood that a brush, a stamp, a spray applicator, a slot-dieapplicator or any other suitable applicator can be used in the place ofthe roller 100. Additionally, as discussed further below, reverse andforward gravure printing, slot die coating, reverse and forward beadcoating and draw down table can also be used to deposit nanowires onto asubstrate. Advantageously, a roller or stamp having recesses of apredetermined pattern can be used to coat a patterned metal nanowiresdispersion layer, or matrix material layer, thus printing a patternedconductive layer (e.g., Gravure printing). The conductive layer can alsobe patterned by spraying the nanowire or matrix formulation onto thesubstrate through an aperture mask. If the matrix material layer isdeposited or cured in a patterned layer, the pattern can be transferredinto the metal nanowire layer by removing sufficient numbers of them todrop the concentration of nanowires below the percolation threshold.Nanowires can be removed by washing or brushing them away with asuitable solvent or by transferring them to a tacky or adhesive roller.

It is further understood that additional depositions or coatings can becarried out, while allowing for drying or curing between two consecutivecoating step. For example, any number of the performance-enhancinglayers can be coated in the same manner as described above.

(b) Web Coating

Web-coating has been employed in the textile and paper industries forhigh-speed (high-throughput) coating applications. It is compatible withthe deposition (coating) processes for transparent conductorfabrication. Advantageously, web-coating uses conventional equipment andcan be fully automated, which dramatically reduces the cost offabricating transparent conductors. In particular, web-coating producesuniform and reproducible conductive layers on flexible substrates.Process steps can be run on a fully integrated line or serially asseparate operations.

FIG. 14A shows an embodiment in which a flexible substrate in the formof a film or web can be coated continuously along a moving path. Morespecifically, a substrate 14 mounted on reels 118 is drawn by a motor(not shown) and moves along a traveling path 120. The substrate can befed to the reels directly or via a conveyor belt system (not shown). Astorage tank 122 is positioned above the substrate 14. The storage tank122 contains a metal nanowires dispersion 124 for metal nanowiresdeposition. An aperture 128 in the storage tank 122 delivers acontinuous stream of metal nanowire dispersion 132 on the substrate 14to form a layer 110 on a top surface 105 of the substrate 14.

It is understood that the matrix material is stored in another storagetank (not shown), and the matrix material can be coated in the samemanner as described above.

It is further understood that any dispensing device can be used in theplace of the storage tank, including a spraying device (e.g., anatomizer that delivers pressurized dispersions), a brushing device, apouring device and the like. Like the sheet coating, a printing devicecan also be used to provide patterned coatings.

FIG. 14B shows an alternative method of web-coating in which the coatingis carried out on a bottom surface of a substrate. Like the methodillustrated in FIG. 14A, a substrate 14 moves along a traveling path120. A coating roller 140 is positioned below the substrate andpartially submerged in a metal nanowire dispersion 124 stored in astorage tank 122. The coating roller 140 delivers a metal nanowiredispersion layer 110 on a bottom surface 144 of the substrate 14.Coating roller 140 can rotate in the direction of the traveling path 120or in the opposite direction. The coating of the matrix material can becarried out in the same manner.

In the processes described in FIGS. 14A and 14B, it is noted thatvarious surface treatments can be applied prior to or after eachdeposition step. As will be described in more detail below, surfacetreatments can enhance the transparency and/or conductivity of theconductive layers formed. Suitable surface treatments include, but arenot limited to solvent or chemical washing, plasma treatments, Coronadischarge, UV/ozone treatment, pressure treatment and combinationsthereof.

FIG. 15A shows a comprehensive process flow for fabricating atransparent conductor. As shown, a web-coating system 146 includes atake-up reel 147 that is driven by a motor (not shown). The take up reel147 draws a substrate 14 (e.g., a flexible polymer film) from a supplyreel 148 along a traveling path 150. The substrate 14 is then subjectedto sequential treatments and coating processes along the traveling path150. It will become apparent to one skilled in the art that the speed ofthe reel, the speed of deposition, the concentration of the matrixmaterial, and the adequacy of the drying and curing processes are amongthe factors that determine the uniformity and the thickness of theconductive layer formed.

Moreover, in certain embodiments, pre-treatments are conducted toprepare the substrate for the subsequent coating processes. Morespecifically, the substrate 14 can be optionally surface-treated at apre-treatment station 160 to improve the efficiency of the subsequentnanowire deposition. In addition, surface treatment of the substrateprior to the deposition can enhance the uniformity of the nanowireslater deposited.

The surface treatment can be carried out by known methods in the art.For example, plasma surface treatment can be used to modify themolecular structure of the surface of the substrate. Using gases such asargon, oxygen or nitrogen, plasma surface treatment can create highlyreactive species at low temperatures. Typically, only a few atomiclayers on the surface are involved in the process, so the bulkproperties of the substrate (e.g. the polymer film) remain unaltered bythe chemistry. In many instances, plasma surface treatment providesadequate surface activation for enhanced wetting and adhesive bonding.As an illustrative example, oxygen plasma treatment can be carried outin a March PX250 system, using the following operating parameters: 150W, 30 sec, O₂ flow: 62.5 sccm, pressure: ˜400 mTorr.

In other embodiments, the surface treatment may include depositing anintermediate layer on the substrate. As noted above, the intermediatelayer typically exhibits affinities for both the nanowires and thesubstrate. Thus, the intermediate layer is capable of immobilizing thenanowires and causing the nanowires to adhere to the substrate.Representative materials suitable as the intermediate layer includemultifunctional biomolecules, including polypeptides (e.g.,poly-L-lysine.)

Other exemplary surface treatments include surface washing with asolvent, Corona discharge and UV/ozone treatment, all of which are knownto one skilled in the art.

The substrate 14 thereafter proceeds to a metal nanowires depositionstation 164, which delivers a metal nanowire dispersion 166, as definedherein. The deposition station can be a storage tank as described inFIG. 14A, a spraying device, a brushing device, and the like. A metalnanowires dispersion layer 168 is deposited on the surface 105.Alternatively, a printing device can be used to apply a patternedcoating of the metal nanowires dispersion on the substrate. For example,a stamp or roller having recesses of a predetermined pattern can beused. The stamp or roller can be continuously dipped into a metalnanowire dispersion by known methods in the art.

The layer 168 can be optionally rinsed at a rinsing station 172.Thereafter, the layer 168 is dried at a drying station 176 to form ametal nanowire network layer 180.

Optionally, the network layer 180 can be treated at a post-treatmentstation 184. For example, surface treatment of the metal nanowires withargon or oxygen plasma can improve the transparency and the conductivityof the network layer 180. As an illustrative example, Ar or N₂ plasmacan be carried out in a March PX250 system, using the followingoperating parameters: 300 W, 90 sec (or 45 sec), Ar or N₂ gas flow: 12sccm, pressure ˜300 mTorr. Other known surface treatments, e.g., Coronadischarge or UV/ozone treatment, may also be used. For example, theEnercon system can be used for a Corona treatment.

As a part of the post-treatment, the network layer can further bepressure treated. More specifically, the network layer 180 is fedthrough rollers 186 and 187, which apply pressure to the surface 185 ofthe network layer 180. It should be understood that a single rollercould also be used.

Advantageously, the application of pressure to a metal nanowire networkfabricated in accordance with a method described herein can increase theconductivity of the conducting layer.

In particular, pressure may be applied to one or both surfaces of aconducting sheet transparent conductor fabricated in accordance with amethod described herein by use of one or more rollers (e.g., cylindricalbars), one or both of which may, but need not, have a length dimensionlarger than a width dimension of the conducting layer. If a singleroller is used, the network layer may be placed on a rigid surface andthe single roller rolled across the exposed surface of the conductinglayer using known methods while pressure is applied to the roller. Iftwo rollers are used, the network layer may be rolled between the tworollers as shown in FIG. 15A.

In one embodiment, from 50 to 10,000 psi may be applied to thetransparent conductor by one or more rollers. It is also considered thatfrom 100 to 1000 psi, 200 to 800 psi or 300 to 500 psi may be applied.Preferably, pressure is applied to a transparent conductor prior to theapplication of any matrix material.

“Nip” or “pinch” rollers may be used if two or more rollers are used toapply pressure to the conducting sheet. Nip or pinch rollers are wellunderstood in the art and discussed in, for example, 3M TechnicalBulletin “Lamination Techniques for Converters of Laminating Adhesives,”March, 2004, which is hereby incorporated by reference in its entirety.

It was determined that application of pressure to a metal nanowirenetwork layer improved the conductivity thereof either before or afterapplication of a plasma treatment as discussed above, and may be donewith or without a previous or subsequent plasma treatment. As shown inFIG. 15A, the rollers 186 an 187 may be rolled across the surface 185 ofthe network layer 180 a single or multiple times. If the rollers arerolled across the network layer 180 multiple times, the rolling may becarried out in the same direction with respect to an axis parallel tothe rolled surface of the sheet (e.g., along the traveling path 150) orin different directions (not shown).

FIG. 15B is an SEM image of a portion of a metal nanowire conductivenetwork 810 after application of from about 1000 psi to about 2000 psiusing a stainless steel roller. Conductive network 810 includes aplurality of nanowire crossing points such as crossing points 812 a, 812b and 812 c. As shown, at least the top nanowires 814, 816, and 818 ateach of crossing points 812 a, 812 b and 812 c have flattened crosssections where the intersecting wires have been pressed into each otherby the application of pressure, thereby enhancing the connectivity aswell as the conductivity of the nanowire conductive network.

The application of heat may also be used at this point as apost-treatment. Typically, the transparent conductor exposed to anywherefrom 80° C. to 250° C. for up to 10 min, and more preferably is exposedto anywhere from 100° C. to 160° C. for anywhere from about 10 secondsto 2 minutes. The transparent conductor can also be exposed totemperatures higher than 250° C. and can be as high as 400° C.,depending on the type of substrate. For example, glass substrate can beheat-treated at a temperature range of about 350° C. to 400° C. However,post treatments at higher temperatures (e.g., higher than 250° C.) mayrequire the presence of a non-oxidative atmosphere, such as nitrogen ora noble gas.

The heating can be carried out either on-line or off-line. For example,in an off-line treatment, the transparent conductor can be placed in asheet-drying oven set at a given temperature for a predetermined amountof time. Heating a transparent conductor in such a way can improve theconductivity of a transparent conductor fabricated as described herein.For example, a transparent conductor fabricated using a reel-to-reelprocess as described herein was placed in a sheet drying oven set at atemperature of 200° C. for 30 seconds. Before this heat post-treatment,the transparent conductor had a surface resistivity of about 12 kΩ/□,which dropped to about 58 Ω/□ after the post-treatment.

In another example, a second, similarly prepared transparent conductorwas heated in a sheet oven at 100° C. for 30 seconds. The resistivity ofthe second transparent conductor dropped from about 19 kΩ/□ to about 400Ω/□. It is also considered that the transparent conductor may be heatedusing methods other than a sheet oven. For example, an infrared lampcould be used as either an in-line or off-line method to heat thetransparent conductor. RF currents may also be used to heat the metalnanowire network. RF currents may be induced in a metal nanowire networkby either broadcast microwaves or currents induced through electricalcontacts to the nanowire network.

Additionally, a post-treatment that applies both heat and pressure tothe transparent conductor can be used. In particular, to apply pressure,the transparent conductor can be placed through one or more rollers asdescribed above. To simultaneously apply heat, the rollers may beheated. The pressure applied by the rollers is preferably from 10 to 500psi and more preferably from 40 to 200 psi. Preferably, the rollers areheated to between about 70° C. and 200° C. and more preferably tobetween about 100° C. and 175° C. Such application of heat incombination with pressure can improve the conductivity of a transparentconductor. A machine which may be used to apply both appropriatepressure and heat simultaneously is a laminator by Banner AmericanProducts of Temecula, Calif. Application of heat in combination withpressure can be done either before or after deposition and curing of amatrix or other layers as described below.

Another post-treatment technique that can be used to increaseconductivity of a transparent conductor is to expose the metal wireconductive network of a transparent conductor fabricated as disclosedherein to a metal reducing agent. In particular, a silver nanowireconductive network can preferably be exposed to a silver reducing agentsuch as sodium borohydride for, preferably, anywhere from about 10seconds to about 30 minutes, and more preferably from about 1 minute toabout 10 minutes. As would be understood by one of ordinary skill in theart, such exposure can be done either in-line or off-line.

As noted above, such a treatment can increase the conductivity of atransparent conductor. For example, a transparent conductor of silvernanowires on a substrate of PET and prepared according to a reel-to-reelmethod disclosed herein was exposed to 2% NaBH₄ for 1 minute, which wasthen rinsed in water and dried in air. Before this post-treatment thetransparent conductor had a resistivity of about 134 Ω/□ and after thispost-treatment, the transparent conductor had a resistivity of about 9Ω/□. In another example, a transparent conductor of silver nanowires ona glass substrate was exposed to 2% NaBH₄ for 7 minutes, rinsed in waterand air-dried. Before this post-treatment the transparent conductor hada resistivity of about 3.3 MΩ/□ and after this post-treatment, thetransparent conductor had a resistivity of about 150 Ω/□. Reducingagents other than sodium borohydride can be used for this posttreatment. Other suitable reducing agents include other borohydridessuch as sodium borohydride; boron nitrogen compounds such as dimethylaminoborane (DMAB); and gas reducing agents, such as hydrogen gas (H₂).

Thereafter, the substrate 14 proceeds to a matrix deposition station188, which delivers a matrix material 190, as defined herein. The matrixdeposition station 188 can be a storage tank as described in FIG. 14A, aspraying device, a brushing device, a printing device and the like. Alayer of the matrix material 192 is thus deposited on the network layer180. Advantageously, the matrix material can be deposited by a printingdevice to form a patterned layer.

The layer 192 is then allowed to cure at a curing station 200. Where thematrix material is a polymer/solvent system, the layer 192 can be curedby allowing the solvent to evaporate. The curing process can beaccelerated by heating (e.g., baking). When the matrix materialcomprises a radiation-curable prepolymer, the layer 192 can be cured byirradiation. Depending on the type of the prepolymer, thermal curing(thermally induced polymerization) can also be used.

Optionally, a patterning step can be carried out before the layer of thematrix material 192 is cured. A patterning station 198 can be positionedafter the matrix deposition station 188 and before the curing station200. The patterning step will be discussed in more detail below.

The curing process forms a conductive layer 204 comprising the metalnanowires network layer 180 in a matrix 210. The conductive layer 204can be further treated at a post-treatment station 214.

In one embodiment, the conductive layer 204 can be surface treated atthe post-treatment station 214 to expose a portion of the metalnanowires on the surface of the conductive layer. For example, a minuteamount of the matrix can be etched away by solvent, plasma treatment,Corona discharge or UV/ozone treatments. Exposed metal nanowires areparticularly useful for touch screen applications.

In another embodiment, a portion of the metal nanowires is exposed onthe surface of the conductive layer 204 following the curing process(see, also, FIGS. 10C and 10D), and an etching step is not needed. Inparticular, when the thickness of the matrix material layer 192 andsurface tension of the matrix formulation are controlled appropriately,the matrix will not wet the top portion of the metal nanowire networkand a portion of the metal nanowires will be exposed on the surface ofthe conductive layer.

The conductive layer 204 and the substrate 14 are then drawn up by thetake-up reel 147. This flow process of fabrication is also referred toas a “reel-to-reel” or “roll-to-roll” process. Optionally, the substratecan be stabilized by traveling along a conveyor belt.

In the “reel-to-reel” process, multiple coating steps can be carried outalong the traveling path of a moving substrate. The web coating system146 therefore can be customized or otherwise adapted to incorporate anynumber of additional coating stations as needed. For example, coatingsof the performance-enhancing layers (anti-reflective, adhesive, barrier,anti-glare, protective layers or films) can be fully integrated into theflow process.

Advantageously, the reel-to-reel process is capable of producing uniformtransparent conductors at high-speed and low cost. In particular, due tothe continuous flow of the coating process, the coated layers do nothave trailing edges.

(c) Lamination

Despite its versatility, the “reel-to-reel” process is not compatiblewith a rigid substrate, such as glass. While rigid substrates can becoated by sheet coating and can possibly be carried on a conveyor belt,they typically experience edge defects and/or lack of uniformity. Inaddition, sheet coating is a lower throughput process, which cansignificantly increase the cost of production.

Thus, it is described herein a lamination process for fabricating atransparent conductor through the use of a flexible donor substrate.This process is compatible with both rigid substrates and flexiblesubstrates. More specifically, the lamination process comprises: coatinga conductive layer on a flexible donor substrate, the conductive layerincluding a plurality of metal nanowires which can be embedded in amatrix; separating the conductive layer from the flexible donorsubstrate; and transferring the conductive layer to a substrate ofchoice. Advantageously, the coating steps onto the flexible donorsubstrate can be carried out by a reel-to-reel process because the donorsubstrate is flexible. The conductive layer formed thereof can then betransferred to a substrate of choice, which can be rigid or flexible,through standard lamination processes. If only nanowires are depositedonto the flexible donor substrate and no matrix material is used, alamination adhesive may be used to attach the conductive layer to thesubstrate of choice.

“Flexible donor substrate” refers to a flexible substrate in the form ofa sheet, film, web, and the like. The flexible donor substrate is notparticularly limited so long as it can be separated from the conductivelayer. The flexible donor substrate can be any of the flexiblesubstrates as described herein. In addition, the flexible donorsubstrate can be woven or non-woven textile, paper, and the like. Theflexible donor substrate need not be optically clear.

In certain embodiments, the flexible donor substrate can be pre-coatedwith a release layer prior to the coating of the conductive layer.“Release layer” refers to a thin layer adhered to the donor substrateand onto which a conductive layer can be formed by web coating. Therelease layer must allow for an easy removal of the donor substrate fromthe conductive layer without damaging the conductive layer. Typically,the release layer is formed of a material having low surface energy,including but not limited to: silicon based polymers, fluorinatedpolymers, starch, and the like.

FIG. 16A illustrates an example of a laminated structure 230 comprising,a flexible donor substrate 240, a release layer 244 coated on theflexible donor substrate 240, and a conductive layer 250 coated on therelease layer 244.

The laminated structure 230 can be fabricated in the same manner asdescribed in connection with FIG. 15A, using the flexible donorsubstrate. Prior to the metal nanowire deposition, the release layer 244is deposited or coated on the flexible donor substrate. The conductivelayer 250 can be formed by metal nanowires deposition followed by matrixdeposition, as described herein.

The conductive layer is then uniformly transferred to a substrate ofchoice. In particular, a rigid substrate (e.g., glass), which istypically not adaptable to the reel-to-reel coating process, can belaminated with the conductive layer. As shown in FIG. 16B, the laminatedstructure 230 is transferred to a substrate 260 (e.g., glass) bycontacting a surface 262 of the conductive layer 250 to the substrate260. In certain embodiments, the polymeric matrix (e.g., PET, PU,polyacrylates) provides adequate adhesion to the substrate 260.Thereafter, as shown in FIG. 16C, the flexible donor substrate 240 canbe removed by detaching the release layer 244 from the conductive layer250.

In other embodiments, an adhesive layer can be used to provide a betterbonding between the conductive layer and the substrate during thelamination step. FIG. 17A shows a laminated structure 270 comprising, inaddition to the flexible donor substrate 240, the release layer 244 andthe conductive layer 250, an overcoat 274 and an adhesive layer 278. Theadhesive layer 278 has an adhesive surface 280.

The laminated structure 270 can be fabricated by a reel-to-reel processas described in connection with FIG. 15A, with the understanding thatthe web coating system 146 is adapted to provide additional stations forcoating the adhesive layer and the overcoat. The adhesive layer is asdefined herein (e.g., polyacrylates, polysiloxanes), and can be pressuresensitive, hot-melted, radiation-curable, and/or thermally curable. Theovercoat can be one or more of the performance-enhancing layers,including a hard coat, an anti-reflective layer, an protective film, abarrier layer, and the like.

In FIG. 17B, the laminated structure 270 is bonded with the substrate260 via the adhesive surface 280. Thereafter, as shown in FIG. 17C, theflexible donor substrate 240 is removed by detaching the release layer244 from the overcoat 274.

In certain embodiments, heat or pressure can be used during thelamination process to enhance the bonding between the adhesive layer (orthe conductive layer in the absence of an adhesive layer) and thesubstrate.

In other embodiments, a release layer is not necessary due to anaffinity differential of the conductive layer with respect to theflexible donor substrate and the substrate of choice. For example, theconductive layer may have a much higher affinity to glass than to atextile donor substrate. After the lamination process, the textile donorsubstrate can be removed while the conductive layer is firmly bondedwith the glass substrate.

In certain embodiments, a patterned transfer is possible during thelamination process. For example, the substrate can be heated by athermal gradient, which affords heated regions and unheated regions onthe substrate according to a predetermined pattern. Only the heatedregions will be laminated with the conductive layer due to an enhancedaffinity (e.g., adhesion), therefore providing a patterned conductivelayer on the substrate. Heated regions on a substrate can be generated,for example, by a nichrome wire heater positioned beneath the areas of asubstrate to be heated.

In other embodiments, a patterned transfer can be affected by a pressuregradient based on a pressure-sensitive affinity displayed by certainmatrix materials or adhesives. For example, a patterned laminatingroller can be used to apply different pressures according to apredetermined pattern. The patterned laminating roller can also beheated to further the affinity differential between pressured region andunpressured region.

In yet other embodiments, the conductive layer can be pre-cut (e.g., diecut) according to a predetermined pattern, prior to the laminationprocess. After transferring the pre-cut conductive layer to thesubstrate, the conductive layer of the predetermined pattern is retainedwhile the rest is removed along a pre-cut contour.

Patterning

As noted above, a patterned conductive layer can be formed byselectively curing a prepolymer coating according to a pattern. Thecuring process can be carried out photolytically or thermally. FIG. 18illustrates an embodiment in which a conductive layer isphoto-patterned. More specifically, the metal nanowire network layer 114is deposited on the substrate 14 according to a method described herein(e.g., FIGS. 13A-13D). It should be understood that the substrate 14 canbe any substrate, including a flexible donor substrate.

Thereafter, a prepolymer coating 300 is deposited on the network layerof metal nanowires 114. An irradiation source 310 provides the photonenergy for curing the prepolymer coating. A mask 314 is positionedbetween the prepolymer coating 300 and the irradiation source 310. Uponexposure, only regions exposed to the irradiation are cured (i.e.,regions 320); the prepolymer coating and nanowires in the uncuredregions 324 can be removed by washing or brushing with a suitablesolvent or by lifting them off with a tacky roller.

Photo-curable prepolymers are well known in the art. In certainembodiments, the photo-curable prepolymer includes a monomer comprisingone or more double bonds or functional groups, e.g. hydrides or hydroxylgroups, suitable for chain extension and crosslinking. In otherembodiments, the photo-curable prepolymer comprises a partial polymer oroligomer that contains one or more double bonds or functional groups,e.g. hydrides or hydroxyls, suitable for cross-linking or chainextension.

Examples of monomers containing a double bond are alkyl or hydroxyalkylacrylates or methacrylates, such as methyl, ethyl, butyl, 2-ethylhexyland 2-hydroxyethyl acrylate, isobornyl acrylate, methyl methacrylate andethyl methacrylate, silicone acrylates, acrylonitrile, acrylamide,methacrylamide, N-substituted (meth)acrylamides, vinyl esters such asvinyl acetate, vinyl ethers such as isobutyl vinyl ether, styrene,alkyl- and halostyrenes, N-vinylpyrrolidone, vinyl chloride andvinylidene chloride.

Examples of monomers containing two or more double bonds are thediacrylates of ethylene glycol, propylene glycol, neopentyl glycol,hexamethylene glycol and of bisphenol A, and4,4′-bis(2-acryloyloxyethoxy)diphenylpropane, trimethylolpropanetriacrylate, pentaerythritol triacrylate or tetraacrylate, vinylacrylate, divinylbenzene, divinyl succinate, diallyl phthalate, triallylphosphate, triallyl isocyanurate or tris(2-acryloylethyl) isocyanurate.

Examples of partial polymers include, but are not limited to,acrylicized epoxy resins, acrylicized polyesters, polyesters containingvinyl ether or epoxy groups, polyurethanes and polyethers, unsaturatedpolyester resins. In a preferred embodiment, the prepolymer is anacrylate. The term “acrylate” refers to a monomer comprising an acryloylmoiety (e.g., methacrylate). “Acrylate” may also refer to a partialpolymer or polymer formed by polymerizing monomers comprising anacryloyl moiety. Examples of the acrylates are as discussed herein.

Optionally, a photo-initiator can be used to initiate the polymerizationand/or crosslinking reactions. The photo-initiator absorbs the photonenergy and produces radicals, which initiates a cascade of radicalpolymerization, including chain-extension and cross-linking.Photo-initiators are well known in the art. Examples of suitablephoto-initiators include, but are not limited to, oxime esters, phenylketones, onium salts, and phosphine oxides, see, e.g. U.S. Pat. Nos.6,949,678, 6,929,896, and 6,803,392; N. Buhler & D. Bellus,“Photopolymers as a powerful tool in modern technology”, Pure & Appl.Chem., Vol. 67, No. 1, pp. 25-31, 1995; J. Crivello in Advances inPolymer Science, Vol. 62, pp. 1-48 (1984). In a preferred embodiment,the photo-initiator is Ciba Irgacure™ 754. Typically, with the use ofthe photo-initiator, the prepolymer coating can cure within 5 minutes,more preferably within 30 seconds.

In other embodiments, thermal-patterning can be carried out using aninsulating thermal mask (e.g., an aperture mask), which only exposesregions of a matrix material layer to be cured to a heat source.Alternatively, in a mask-less approach, laser direct-write technologycan be used to directly “write” a heated pattern on the prepolymercoating layer. Thermally-curable matrix materials are known to oneskilled in the art. For example, the matrix material can be an epoxy, aresin and a sol-gel composite material.

Both the photo-patterning method and thermal-patterning method arecompatible with the “reel-to-reel” process described above. For example,a photo-patterning station 198 can be a part of the web coating system146, as shown in FIG. 15A. The photo-patterning station 198 can beconfigured in a number of ways to allow for continuous exposure andcuring of the prepolymer coating.

In one embodiment, as shown in FIG. 19A, a rotating cylinder 330 is partof the photo-patterning station 198 (the web coating system 146 is notshown). The substrate 14, coated with a prepolymer coating 300, is movedalong by a conveyor belt 332. The rotating cylinder rotates at the samespeed as the conveyor belt 332. The irradiation source 310 is positionedwithin the rotating cylinder 330. An exterior 334 of the rotatingcylinder 330 is patterned, perforated or otherwise provided withopenings 338 to allow the light to irradiate the prepolymer coating 300.Optionally, a guard slit or a collimator 340 for preventing any straylight can be positioned closely above the moving substrate.

In a related configuration, as shown in FIG. 19B, a patterning belt 350having a patterned or perforated exterior 352 can be used. Thepatterning belt 350 is driven by rollers 354, one of which is connectedto a motor (not shown). The patterning belt 350 moves at the same speedas the moving conveyor belt 332, allowing continuous exposure of theprepolymer coating 300 to the irradiation source 310 through openings360. Optionally, a guard slit 340 can be used.

FIG. 20 shows a partially integrated system 400 for forming a patternedconductive layer on a substrate. The system 400 can be fully integratedinto the web coating system 146. In particular, the photo-patterningstation 198 is identical to the one shown in FIG. 19A. Following thephoto exposure and curing, the prepolymer coating 300 is cured atselective regions and will be further treated at a washing station 370to remove any uncured prepolymer. The substrate 14, now comprising curedregions 380 and bare metal nanowires regions 374, moves to a rotatingtacky roller 384. The tacky roller 384 contacts and removes the baremetal nanowires regions 374. Following the removal of the bare metalnanowires, the substrate is coated with conductive regions 380 amongnon-conductive regions 386.

In a further embodiment, a conductive layer can be patterned by etching.Depending on the composition of the conductive layer, various etchingsolutions can be used to dissolve and remove portions of the conductivelayer in the unmasked areas.

In one embodiment, an acid-etching solution comprising nitric acid(HNO₃) can be used. Typically, the nitric acid is present at 0.01-40%,more typically, the nitric acid is present at 0.01-10%. The acid-etchingsolution may further comprise a trace amount (e.g., about 1-100 ppm) ofpotassium permanganate (KMnO₄). In one embodiment, the acid-etchingsolution comprises about 1% HNO₃, 1% NaNO₃ and a trace amount (severalparts per million) of potassium permanganate (KMnO₄). The etchingsolution converts metal nanowires to soluble metal salt, which can beremoved by washing. For example, silver nanowires can be converted tosilver salt (Ag⁺), which can be rinsed off by a solvent, e.g., water.

In certain embodiments, the etching solution does not affect or dissolvea matrix composed of fully or partially cured polymer. Patterning can becarried out by depositing and curing a polymeric matrix material on ananowire layer according to a desired pattern. Once the matrix is cured(either fully or partially) to attain an acceptable degree of hardnessand physical form, the matrix protects the nanowires embedded thereinfrom being etched away during a subsequent etching step. The nanowiresin the unprotected areas (where the matrix material is not polymerizedor where no matrix is present) can be etched and removed. Thus, oneembodiment describes a method of patterning, the method comprising:forming a conductive layer on a substrate, the conductive layercomprising a plurality of nanowires; forming a matrix on the conductivelayer according to a pattern, the pattern comprising nanowires protectedby a matrix and unprotected nanowires; and etching the conductive layerto dissolve the unprotected nanowires. The unprotected nanowires can beeither removed or left in place.

It has been found that the amount of KMnO₄ in the acid-etching solutiondescribed herein can impact the etching power. For example, the amountof KMnO₄ in the acid etching solution can affect the rate of etching.Typically, higher concentration of KMnO₄ results in faster etching.Accordingly, adjusting the concentration of KMnO₄ in the acid etchingsolution can modulate the etching efficiency without changing theacidity of the etching solution.

It has also been observed that higher concentration of KMnO₄ in the acidetching solution may cause more effective diffusion of the etchingsolution into the matrix, which results in faster or more completedissolution of the nanowires in situ. For example, as shown in Examples9, 10 and 11, when KMnO₄ is present in the etching solution at less than20 ppm, the matrix (of a standard thickness of about 150 nm) can protectthe nanowires embedded therein from being etched. When the amount ofKMnO₄ is increased to about 20 ppm, while the concentrations of HNO₃ andNaNO₃ remain constant, the etching solution diffuses into the matrix(about 150 nm thick) and dissolves the nanowires embedded therein.

As will be discussed in more detail in connection with Example 11, athick overcoat (about 1 μm) can effectively prevent the diffusion of anacid-etching solution and protect the nanowire from being etched, whilethe nanowire/matrix unprotected by the thick overcoat was dissolved bythe acid-etching solution (e.g., 20 ppm KMnO₄, 1% HNO₃ and 1% NaNO₃.)

Thus, in one embodiment, an etching solution can be selected that iscapable of diffusing into the matrix and dissolving the nanowires. Inthese embodiments, conductive layers comprising nanowires in a matrixcan be etched by using a protective mask (e.g., a photoresist). Thus,patterning can be carried out according to standard photolithographymethods, by which the nanowires in the unmasked region are etched.

In a further embodiment, etching the unmasked region comprises etchingthe matrix in the unmasked region using a first etchant; and etching thenanowires in the unmasked region using a second etchant. For example, afirst etchant (e.g., hydrogen peroxide) can be used to remove the matrixto expose or deprotect the nanowires in the unmasked region. Thereafter,a second etchant, such as the acid-etching solution discussed herein,can be used to dissolve or remove the nanowires that are no longerprotected by the matrix.

Thus, other embodiments describe a method of patterning the transparentconductor using a mask. The mask acts as a thick overcoat, protectingthe nanowire/matrix layer underneath. The method comprises: forming aconductive layer on a substrate, the conductive layer comprising amatrix and a plurality of electrically conductive nanowires embeddedtherein; placing a mask on the conductive layer to define a maskedregion and an unmasked region; and etching the unmasked region using anacid-etching solution to form a patterned conductive region. The methodmay further comprise removing the etched region to form a pattern.

Other factors that can contribute to etching efficiency include, but arenot limited to, the degree of curing of the matrix. For example, giventhe same etching solution and same monomers, a matrix formed bypartially cured polymer tends to dissolve more readily than a matrixformed by fully cured polymer. After patterning, the partially curedmatrix may undergo an additional curing step to fully cure the matrix.

More efficient etching can also be achieved by activating an etchingsurface of the transparent conductor prior to etching. Such apre-treatment is particularly beneficial to a wet-etching process, inwhich a liquid etchant comes into contact with the etching surface ofthe transparent conductor. Typically, the etching surface of thetransparent conductor can be a top surface of the nanowire/matrix layeror, in some instances, a top surface of an overcoat layer. The matrixlayer and the overcoat layer protect the underlying nanowires fromcorrosive elements and abrasion. Their presence, however, may cause poorwetting of the liquid etchant. Pre-treating the etching surface of thetransparent conductor can activate the surface and improve its wettingbehavior. As a result, the liquid etchant can gain access to the metalnanowires protected by the matrix and/or the overcoat layer.

Thus, the method of patterning described above can further comprisepre-treating an etching surface of the conductive layer to improve itswetting behavior.

The change in the wetting behavior can be assessed by water contactangle measurements. Water contact angle refers to the angle at which aliquid/vapor interface meets a solid surface (i.e., the etchingsurface). Typically, a higher water contact angle is correlated withpoorer wetting of the solid surface. As shown in Table 1, depending onthe types of treatment, the water contact angles are substantiallyreduced by about 50% to 80% after surface treatments.

TABLE 1 Water Contact Angle (°) Surface Treatment without surfacetreatment With surface treatment Oxygen Plasma 62.4 12.5 UV ozone 63.534.5

As further described in detail in Example 13, using etchants of the samestrength, the rate of etching surface-treated transparent conductors issignificantly improved as compared to the rate of etching untreatedtransparent conductors.

Accordingly, transparent conductor films can be efficiently patterned bypre-treating the regions to be etched.

Moreover, by adjusting the types and/or strength of the etchants, it ispossible to create a patterned transparent conductor film withsubstantially uniform optical properties or low visibility patterns.These transparent conductors with low-visibility patterns areparticularly useful as components in displays such as touch screens.

As used herein, “substantially optically uniform,” “optically uniform,”or “optical uniformity” refers to an optical characteristic of apatterned transparent conductor film having at least two regions ofdistinctive conductivities, the ratio of the resistivity of the etchedregion over the resistivity of the unetched region being at least 10³,or at least 10⁴, or at least 10⁵, or at least 10⁶, wherein thedifference between the light transmissions (T %) of the two regions isless than 5%, or less than 4%, or less than 3%, or less than 2%, or lessthan 1%, or less than 0.5% or zero; and wherein the difference betweenthe haze values (H %) of the two regions is less than 0.5%, less than0.1%, less than 0.07%, less than 0.05%, less than 0.03%, less than 0.01%or zero. This process, which minimizes or eliminates the opticaldifferences between etched and unetched regions of a patternedconductive film, is also referred to as “low-visibility patterning.” Inthese incidences, it is believed that the nanowires are not completelyetched away or dissolved, yet the nanowire network in the etched regionhas been rendered less conductive than those in the unetched region. Incontrast with the “low-visibility patterning” described herein, anetching process that substantially or entirely dissolves the conductivemedium (e.g., metal nanowires) results in a substantially lowered hazein the etched region due to a reduction of light scattering, in whichcase the optical differences between the etched and unetched regions canbe substantial enough that the etched pattern is more clearly visible.

Thus, in a further embodiment, a conductive layer can be patterned bycreating a non-conductive region, or region having an alteredresistivity that is not necessarily non-conductive, without completelydestroying or removing the nanowires. In this way, any change in opticalcharacteristics (i.e., transmission and haze) of the etched region canbe relatively minimal, yet the resistivity of the conductive medium(i.e., the interconnected nanowire network) is altered in the etchedregions according to a predetermined pattern. More specifically, thisembodiment provides a method comprising: forming a conductive layer on asubstrate, the conductive layer comprising a matrix and a network ofelectrically conductive nanowires embedded therein; and treating aregion of the conductive layer to alter the resistivity of the networkof electrically conductive nanowires within the region, thereby forminga patterned conductive layer including a treated region having a firstresistivity and an untreated region having a second resistivity. Thetreated region may, but need not be, rendered non-conductive. As usedherein, “non-conductive” refers to a surface resistivity of at least 10⁶Ω/□. In certain embodiments, a ratio of first resistivity over thesecond resistivity is at least 1000 (i.e., the etched region is lessconductive than the unetched region). In other embodiments, a ratio offirst resistivity over the second resistivity is at least 10⁴, or atleast 10⁵, or at least 10⁶.

In addition, disclosed is a transparent conductor including a substrateand a patterned conductive layer having a network of electricallyconductive nanowires embedded in a matrix. The patterned conductivelayer defines a first region of the transparent conductor in which thenetwork has a first resistivity and a second region of the transparentconductor in which the network has a second resistivity. The change ordifference in optical characteristics (i.e., transmission and haze)between the two regions is relatively small. For example, withoutlimitation, the difference in the transmission and haze, respectively,of the first region differs from that of the second region by less than0.7% and 0.62%, respectively, while the change in resistance between thetwo regions is greater than about 1500 Ω/□.

As described herein, an optically clear conductive layer can be treatedor etched according to the patterning method described herein withoutaffecting the optical properties of the treated or etched region. Asfurther illustrated Example 14, the change in optical properties,including transmission (T %) and haze (H %), was relatively small beforeand after the patterning step such that the etched patterns obtainedhave low visibility. In such an “invisible patterning” or “lowvisibility patterning” method, the conductive layer remains opticallyuniform in appearance; but is conductive in the untreated (or un-etched)region according to a predetermined pattern, the treated or etched areashaving been rendered nonconductive or having a different and lowerconductivity. In the present embodiment, altering the resistivity of thenanowire network can be accomplished by, without limitation, destroyingor degrading the electrical conductivity of the connections between thenanowires or rendering the nanowires themselves non-conductive. In oneembodiment, treating the electrically conductive nanowire networkcomprises chemically transforming the electrically conductive nanowiresto non-conductive nanowires or wires having higher resistivity. Suchchemical transformation may include, for example, oxidation,sulfidation, or any other process that converts the underlyingconductive material of the nanowire to an electrically insulatingmaterial. For example, conductive nanowires formed by elemental metal ormetal alloys (e.g., silver) can be rendered non-conductive when themetal is converted to an electrically insulating and insoluble metalsalt (e.g., silver chloride.) In this example, the elemental silver canbe initially oxidized and converted to silver ion (Ag⁺). The oxidationcan be further driven to completion in the presence of an anion (e.g.,Cl⁻), with which the positively charged silver ion can form an insolublesalt (AgCl). Other examples of anions that readily precipitate metalions into insoluble metal salts include, for example, bromide, iodideand sulfate.

Examples of suitable oxidizing agents include, but are not limited to,peroxides (e.g., hydrogen peroxide), persulfates (e.g., ammoniumpersulfate), peroxo compounds (e.g., sodium or potassiumperoxodisulfate), halogens or halogen-based oxidizing salts (e.g.,chlorine or hypochlorite salts), oxidizing metal salts (e.g., palladium,manganese, cobalt, copper or silver salts), organic oxidizing agentssuch as 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), and gaseous oxidizingagents such as air, oxygen, ozone and the like.

In various embodiments, and as illustrated in Examples 14 and 15 below,the concentration of the oxidizing agent, type of agent and time ofexposure to the agent may determine the extent of the chemicaltransformation of the conductive layer. It is possible that a strongand/or more concentrated oxidizing agent may cause the nanowires and thematrix layer to dissolve (see, e.g., Example 14.)

In certain embodiments, in addition to, or instead of, transforming theunderlying material of the nanowires from conductive to less conductiveor non-conductive, it is also possible that the nanowires may bephysically compromised. For example, the nanowires may become broken orshortened, thereby reduce the level of their interconnectivity. As aresult, the overall resistivity of the treated region increases comparedto the untreated region, in part due to the formation of theelectrically insulating material and in part due to the breakdown of theinterconnectivity among the nanowires. It is noted that such changes inthe physical structures of the nanowires may occur on a microscopiclevel only, thus would not affect the macroscopic appearance (e.g., theoptical properties) of the conductive layer. Accordingly, the conductivelayers described herein may be treated to form conductive patterns thatare optically uniform, as defined herein.

One embodiment describes a patterned, optically uniform conductive filma substrate; a conductive film on the substrate, the conductive filmincluding a plurality of interconnecting nanostructures, wherein apattern on the conductive film defines (1) an unetched region having afirst resistivity, a first transmission and a first haze and (2) anetched region having a second resistivity, a second transmission and asecond haze; and wherein the etched region is less conductive than theunetched region, and a ratio of the first resistivity over the secondresistivity is at least 1000; the first transmission (T %) differs fromthe second transmission (T %) by less than 5%; and the first haze (H %)differs from the second haze (H %) by less than 0.5%.

In other various embodiments, the ratio of the first resistivity overthe second resistivity is at least 10⁴, or at least 10⁵, or at least10⁶.

In certain other embodiments, the first transmission (T %) differs fromthe second transmission (T %) by less than 4%, or less than 3%, or lessthan 2%, or less than 1%, or less than 0.5% or zero.

In further embodiments, the first haze (H %) differs from the secondhaze (H %) by less than 0.1%, less than 0.07%, less than 0.05%, lessthan 0.03%, less than 0.01% or zero.

An etchant is selected to create small cuts in metal nanostructures(e.g., silver nanowires) such that at least some nanostructures arebroken down to shorter segments. Although the small cuts undermine theconductivity of the nanostructures and render the conductive film in theetched region non-conductive or less conductive, they do notsignificantly alter the optical properties (T % and/or H %) of theetched region as compared to those of the un-etched region.

Suitable etchants can be, for example, an aqueous solution of a metalchloride salt, such as copper (II) chloride (CuCl₂) or iron (III)chloride (FeCl₃). The etchant may further include a strong acid, such ashydrogen chloride or nitric acid. The respective concentrations of theetchant components can be adjusted. Typically, higher concentrations ofthe metal salt and/or the strong acid lead to higher rates of etching.In certain embodiments, an etchant solution includes (by w/w %) about 12to 24% CuCl₂ and about 1.4 to 6.8% of HCl, and the remainder of theetchant solution is water. In other embodiments, an etchant solutionincludes w/w %) about 30% FeCl₃ and about 4% of HCl.

The extent of etching can be determined by the resistivity of the etchedregion. The etched region is generally less conductive than the unetchedregion. For example, the resistivity of the etched region can be atleast twice, at least 5 times, at least 10 times, at least 100 times, atleast 1000 times, at least 10⁴ times, at least 10⁵ times or at least 10⁶times of the resistivity of the unetched region. Various extents ofetching can be controlled through controlling the etching time and/orthe strength (i.e., concentration) of the etchant. The temperature atwhich the etching takes place can also affect the rate and extent ofetching. Typically, etching at higher temperatures results in higheretching rates. In various embodiments, the etching is carried out atbetween about 20 and 60° C.

In another embodiment, the low-visibility patterning process comprisesan initial partial etching step and a subsequent heating step. Morespecifically, the process comprises:

etching the conductive film according to a pattern to provide (1) anunetched region having a first intermediate resistivity, and (2) anetched region having a second intermediate resistivity, wherein a firstratio of the first intermediate resistivity over the second intermediateresistivity is less than 1000; and

heating the conductive film such that the etched region has a firstfinal resistivity and the unetched region has a second finalresistivity, wherein a second ratio of the first final resistivity overthe second final resistivity is at least 1000, and wherein the etchedregion and the unetched region are optically uniform.

In certain embodiments, the first ratio of intermediate resistivitiesfollowing the partial etching step is less than 5, or less than 2,whereas the second ratio of final resistivities following the heatingstep is at least 10⁴, or at least 10⁵ Ω/□, or at least 10⁶. Thus, if tobe combined with a heating step, the initial etching step may partiallyetch only to the extent that the subsequent heating step can completethe etching process. Compared to a complete etching, which etches to befully non-conductive (e.g., having a resistivity of at least 10⁶ Ω/□), apartial etching causes less damage to the nanostructures, as reflectedby lower intermediate resistivity than those of the final resistivity.As a result, a first ratio in resistivities between the (partially)etched and unetched regions is substantially smaller than what acomplete etching would have accomplished (i.e., the second ratio).Nevertheless, the nanostructures are sufficiently damaged by the partialetching that they could be completely severed by the subsequent heatingprocess, resulting in a second ratio of resistivities between the etchedand unetched region that is larger than the first ratio.

This “etching by heating process” is likely due to an increasedsensitivity of nanostructures to melting behavior as their dimensionsget smaller (i.e., thinner) from the partial etching step. For example,the etching by the CuCl₂ solution decreases the nanowire diameter invery small areas along the length of the wires, and the nanowires thusdamaged are particularly susceptible to complete break-downs by the“etching by heating process.”

It is unexpected that the heating step causes the partially etchedregion less conductive or non-conductive since generally a heating stephas the opposite effect on film conductivity. As described herein, aheating step may be employed as a post-treatment step to increase thefilm conductivity of a given sample. In a partially etched film, on theother hand, the damage created by the partial etching unexpectedlychange the sensitivity of nanostructures to melting behaviors, whichrenders the nanostructures susceptible to breakage, resulting in adecrease of film conductivity.

Advantageously, the etched and unetched regions of the patternedconductive film prepared by the “etching heating process” are opticallyuniform, resulting in low-visibility patterns. It is believe that thepartial etching thins the nanostructures to a lesser extent than acomplete etching, thus resulting in less change in haze. Further, theadditional heating step makes the process more robust. For instance, iffor any reason the initial etching step is less effective than expected,the subsequent heating step can complete the process of making theetched region of the conductive film less conductive or non-conductive.

Optionally, a pre-treatment step can be carried out prior to forming aninvisible pattern or low-visibility pattern by improving the wettingbehavior of the etching surface and changing the surface energy. Asdiscussed, oxygen plasma and UV ozone are examples of suitablepre-treatments.

Applications of the Nanowire-Based Transparent Conductors

The transparent conductors as described herein can be used as electrodesin a wide variety of devices, including any device that currently makesuse of transparent conductors such as metal oxide films. Examples ofsuitable devices include flat panel displays such as LCDs, plasmadisplay panels (PDP), color filters for colored flat panel displays,touch screens, electromagnetic shielding, functional glasses (e.g., forelectrochromic windows), optoelectronic devices including EL lamps andphotovoltaic cells, and the like. In addition, the transparentconductors herein can be used in flexible devices, such as flexibledisplays and touch screens.

(a) Liquid Crystal Display

An LCD is a flat panel display that displays an image by controllinglight transmissivity by an external electric field. Typically, the LCDincludes a matrix of liquid crystal cells (or “pixels”), and a drivingcircuit for driving the pixels. Each liquid crystal cell is providedwith a pixel electrode for applying an electric field to the liquidcrystal cell with respect to a common electrode. If each of the pixelelectrodes is connected to a thin film transistor (TFT) together theyfunction as a switching device, i.e., the pixel electrode drives theliquid crystal cell in accordance with a data signal applied via theTFT.

The TFT LCD panel comprises two substrates with the liquid crystal cellsinterposed in between. The pixel electrodes are provided on a lowersubstrate for each liquid crystal cell, whereas the common electrode isintegrally formed on the entire surface of an upper, opposing substrate.The lower substrate, also referred to as a TFT array substrate or TFTbackplane, thus comprises an array of thin film transistors connected tocorresponding pixel electrodes. The upper opposing substrate, comprisesthe common electrode which may be coated on a color filter whichcombination may be referred to as the color filter substrate.

Conventionally, the pixel electrode is made of a highly transmissive ITOfilm in order to allow sufficient light to transmit through. As notedabove, ITO films are costly to fabricate and may be susceptible tocracking if used on a flexible substrate. The nanowire-based transparentconductor films described herein offer an alternative approach in TFTpixel electrode fabrication.

Generally speaking, the thin film transistors described herein can befabricated according to any known methods in the art. The nanowire-basedpixel electrode can be formed by coating the TFT back-plane with ananowire transparent conductor film, followed by a patterning step.Alternatively, a patterned transparent conductor layer can be preformedprior to coating. Patterning of the nanowire-based transparent conductorfilm can also be achieved according to the high throughput methodsdescribed herein.

In addition to being patternable, the nanowire-based transparentconductor films also have the conformability and high temperatureresistance required for TFT LCD manufacturing and applications, i.e., aspart of a TFT-based switching device.

FIG. 21A is an SEM image showing the conformality of a nanowire-basedtransparent conductor film over a substrate with a profile or relieffeature in the surface thereof. As shown, the transparent conductor film387 at least partially conforms to a groove 388 on the substrate. Thesubstrate is a color filter and groove 388 has a width of about 2 μm to3 μm. Larger or smaller width grooves may also be conformally coveredwith a transparent conductor film. For example, grooves having a widthdown to 0.1 μm to 2 μm and up to 3 μm or greater can be coatedconformally. Additionally, substrates having profiles or relief featuresother than grooves may be at least partially conformally coated with ananowire-based transparent conductor film. For example, and withoutlimitation, a profile or relief feature could include a step, ramp,shoulder and/or a concave or convex curb. FIG. 21B shows a magnifiedimage of FIG. 21A. As shown, the nanowire network remains intact andconforms to the groove 388. FIGS. 21A-21B illustrate that nanowire-basedtransparent conductor films are sufficiently conformal with a surfacetopology of, for example, a TFT. That a nanowire network at leastpartially conforms to a profile or relief feature of a substrateadvantageously allows conductivity across such a profile or relieffeature and may be more robust that a conductive layer that would simplybridge a profile or relief feature without conforming thereto.Nanowire-based transparent conductor film can withstand the temperatureconditions during fabrication and operation of the LCD. Morespecifically, their optical and electrical properties show relativelylittle fluctuation. FIG. 22 is a graphic representation of thetemperature resistance of nanowire-based transparent conductor film. Theoptical and electrical properties (e.g., haze, transmission and sheetresistance) were measured at time (t) on the X axis. The retentions[Y(t)/Y(0)], shown as 389, 390 and 391 for haze, transmission and sheetresistance, respectively, were plotted against the time (t). All threelines are nearly linear and have slopes close to 1, indicating that theretentions of the optical and electrical properties remain nearlyconstant at high temperature within the tested time frame.

The nanowire-based conductive film is also a suitable material forforming via contacts in a LCD-TFT plate due to its ability toconformally coat a surface with a profile or relief feature. For atypical LCD-TFT plate, vias are etched in the passivation layer SiNx toconnect the drain. Currently, sputtered ITO films are used to establishvia contact. The sputter ITO film can, however, be replaced by thenanowire-based conductive film described herein.

A typical process comprises spin-coating a metal nanowire dispersion(e.g., silver nanowire, HPMC and Zonyl®) into a via followed by apost-baking step, e.g., at 180° C. for 90 seconds. Typically, a furtherpost-treatment is needed to establish via contacts. Exemplarypost-treatments include one or more of the following methods: runninghigher current through the coated vias, rinsing the coated vias withdeionized (DI) water rinsing, treating with argon (Ar) plasma or UVozone, coating with additional conductive materials, and adjusting wiredensity or dimensions.

In particular, nanowire-coated vias rinsed with DI water or treated withAr plasma are shown to have established contacts in vias as small as 5μm. It is believed that rinsing or plasma treatment cleans the surfaceof the electrode at the bottom of the via as well as the nanowires,thereby providing a more pristine contact between the electrode and thenanowires and reducing contact resistance.

The nanowire-based transparent conductor films are compatible with allthe TFT configurations currently used in the LCD technology. In general,thin film transistors fall into two broad categories: a bottom-gate typeand a top-gate type. In the bottom-gate TFT, a gate electrode isdisposed below an active layer, whereas in the top-gate TFT, a gateelectrode is disposed above an active layer. The bottom-gate thin-filmtransistors typically have superior reliability in comparison with thetop-gate thin-film transistor. These structural configurations aredescribed in more detail in, for example, Modern Liquid Crystal ProcessTechnologies '99 (Press Journal, 1998, pp. 53 to 59) and Flat PanelDisplay 1999 (Nikkei BP, 1998, pp. 132 to 139). Moreover, depending onthe type of material that forms the active area, thin film transistorscan also be based on amorphous silicon, polycrystalline silicon andorganic semiconductors.

FIG. 23 shows the cross-sectional view of a switching device of a TFTbackplane according to one embodiment. As shown, the switching device394 comprises a bottom-gate thin film transistor 396 and ananowire-based pixel electrode 398. The thin film transistor includes agate electrode 400 formed on a substrate 402. The gate electrode can bea metal layer (e.g., Mo—Al—Cd) defined by photolithography. A gateinsulating layer 406 overlies the gate electrode 400. The thin filmtransistor 396 further includes an insulating layer 410, a firstsemiconductor layer 414 (e.g., amorphous silicon) and a secondsemiconductor layer 418 (e.g., n+ doped amorphous silicon), all definedto form an island-shaped structure. A source electrode 422 and a drainelectrode 426 define a channel 430, exposing a portion of the firstsemiconductor layer 414 (i.e., active layer). A further protective layer434 covers the island structure, the source and drain electrodes whileexposing a contact hole 438. The protective layer 434 is, for example, asilicon nitride layer. A nanowire-based transparent conductor film 442is coated over the thin film transistor 396. The nanowire-basedtransparent conductor film 442 can be deposited and patterned asdescribed herein, to form the pixel electrode 398. In other portions ofthe TFT backplane, the same nanowire-based transparent conductor film442 can also be patterned to define a signal line area 446.

In a further embodiment, the switching device described above can beincorporated in a liquid crystal display (LCD) device.

FIG. 24 shows schematically an LCD device 500 comprising a TFT backplane501 and a color filter substrate 502. A backlight 504 projects lightthrough a polarizer 508 and a glass substrate 512. A plurality of firsttransparent conductor strips 520 are positioned between the bottom glasssubstrate 512 and a first alignment layer 522 (e.g., a polyimide layer).Each transparent conductor strip 520 alternates with a data line 524.Spacers 530 are provided between the first alignment layer 522 and asecond alignment layer 532, the alignment layers sandwiching liquidcrystals 536 in between. A plurality of second transparent conductorstrips 540 are positioned on the second alignment layer 532, the secondtransparent conductor strips 540 orienting at a right angle from thefirst transparent conductor strips 520. The second transparent conductorstrips 540 are further coated with a passivation layer 544, a colorfilter of colored matrices 548, a top glass substrate 550 and apolarizer 554. Advantageously, the transparent conductor strips 520 and540 can be patterned and transferred in a laminating process onto thebottom glass substrate, and the alignment layer, respectively. Unlikethe conventionally employed metal oxide strips (ITO), no costlydeposition or etching processes are required.

FIG. 25 shows a cross-sectional view of a LCD based on a top-gate TFTaccording to another embodiment. As shown, the LCD 542 has a TFTsubstrate 544 and a color filter substrate 546 with a liquid crystallayer 548 interposed between them. As noted above, in the TFT substrate544, thin film transistors 550 and pixel electrodes 552 are arranged ina matrix configuration on a bottom transparent substrate 554. A commonelectrode 556, to which a common voltage can be supplied and a colorfilter 558 are disposed on a top transparent substrate 560. A voltageapplied between the pixel electrode 552 and the common electrode 556,which are facing each other with the liquid crystal 548 between them,drives the liquid crystal cells (pixels).

The thin film transistor 550 disposed for each of the pixels on thebottom transparent substrate 554 is a top-gate type TFT, whose gateelectrode 562 is located above an active layer 564. The active layer 564of the TFT is patterned on the bottom substrate 554 according to knownmethods in the art. A gate insulating layer 566 overlies and covers theactive layer 564. The part of the active layer 564 facing the gateelectrode 562 is a channel region 564 c. A drain region 564 d and asource region 564 s with an impurity doped are positioned at respectivesides of the channel region 564 c. The drain region 564 d of the activelayer 564 is connected to a data line, which functions also as a drainelectrode 566, through a contact hole formed in an interlayer insulatinglayer 568 covering the gate electrode 562. Also, an insulating layer 570is disposed to cover the data line and the drain electrode 566. Ananowire-based transparent conductor film forming the pixel electrode552 is positioned on the insulating layer 570. The pixel electrode 552is connected to the source region 564 s of the active layer 564 througha contact hole. A first alignment layer 572 may be positioned on thepixel electrode.

FIG. 25 further shows a storage capacitance element 574, which can bedisposed for each pixel. The storage capacitance element maintains theelectric charge corresponding to the display contents, which should beapplied to the liquid crystal capacitance, when the TFT is not selected.Therefore, the voltage change of the pixel electrode 552 can bemaintained, enabling the display contents to be kept unchanged duringone sequence.

As shown, the source region 564 s of the active layer 564 functions alsoas a first electrode 576 of the storage capacitance element 574. Asecond electrode 578 of the storage capacitance element 574 can beformed simultaneously with and in the same layer as the gate electrode562. The gate insulating layer 566 also works as a dielectric betweenthe first electrode 576 and the second electrode 578. The gate electrode566 (i.e., gate line) and the second electrode 578 (i.e., a storagecapacitance line) are arranged in parallel. They are oriented at a rightangle from the pixel electrode 552 to define the matrix of pixels.

It should be understood that for both of the bottom-gate and top-gateTFT configurations, the active layer can be any acceptable semiconductormaterial. Typically, amorphous silicon is widely used due to the easeand economy of the deposition and patterning steps. Polycrystallinesilicon can also be used. Because polycrystalline silicon has bettercurrent-driving capability than amorphous silicon, it provides superiorperformance when used in a switching device. Low temperature depositionof polycrystalline silicon is possible and has been reported as analternative approach to manufacturing polycrystalline silicon-based TFT,see, e.g., U.S. Pat. No. 7,052,940. In addition, organic semiconductormaterial can also be used. In certain embodiments, an organic πconjugate compound can be used as the organic semiconductor materialthat forms the active layer of an organic TFT. The π conjugate compoundsare known in the art, which include without limitation: polypyrroles,polythiophenes (may be optionally doped with C₆₀), polypyrenes,polyacetylene and polybenzothiophenes, and the like. More examples ofthe organic semiconductor materials suitable for the organic TFTs aredescribed in, for example, U.S. Pat. No. 7,018,872.

As discussed herein, the TFT backplane is positioned in an LCD oppositeto a color filter substrate (see, e.g., FIGS. 24 and 25). The colorfilter substrate typically comprises a transparent substrate, a blackmatrix (or a light-shielding layer) and an array of colored pixels.Typically, the colored pixels are arranged on the transparent substratein a pattern. The black matrix forms a grid around each colored pixel.In certain embodiments, each colored pixel is associated with a color.In other embodiments, each colored pixel can be further divided intosmaller colorant areas (referred to as subpixels), each subpixel beingassociated with a color. Typically, primary colors such as red (R),green (G) and blue (B) are used. For example, repeating arrays of RGBtriads are capable of producing color images of a wide variety ofcolors. The colored pixels or subpixels are not limited to primarycolors, other colors such as white, yellow or cyan can also be used.

In addition, the color filter substrate comprises a common electrode,which is a reference electrode to the pixel electrode in the TFTbackplane. In certain embodiments, the common electrode can be formed ofthe nanowire-based transparent conductor as described herein.

Depending on the mode of an LCD, the relative positions of the commonelectrode and the unit color filters can be different in a TN (twistednematic) mode from that in an IPS (In-plane-switching) mode.

FIG. 26A shows a cross-sectional view of a color filter substrate in aTN mode LCD according to one embodiment. The color filter substrate 580includes a transparent substrate 582 having a displaying surface 584.The transparent substrate 582 can also be referred as a top or uppertransparent substrate due to its spatial relation to the TFT backplanein a final LCD configuration (e.g., 560 in FIG. 25). The color filtersubstrate 580 further includes a black matrix (or light-shielding layer)586 disposed in a lattice shape, and is formed on the transparentsubstrate 582 opposite from the displaying surface 584. External lightis reflected by the black matrix and comes out of the displaying surface584. The black matrix 586 can be made of a metal oxide and/or metal film(e.g., chrome/chrome oxide). Alternatively, the black matrix can be madeof organic resins.

The color filter substrate 580 may comprise a plurality of coloredpixels, such as first, second and third colored pixels 588 a, 588 b and588 c for transmitting red (R), green (G) and Blue (B) color lights,respectively. They are formed on at least one portion of the transparentsubstrate 582 free of the black matrix 586. The first, second and thirdcolored pixels 588 a, 588 b and 588 c are made from, for example, anacrylic resin or polyimide group resin dispersed with pigments, and isformed separately from the black matrix 586 to prevent color-mixing. Inaddition to the primary colors (e.g., RGB), the color filter substratesmay contain other colors, for example RGBW or RG1G2B.

A common electrode 590 formed of a nanowire-based transparent conductorfilm is positioned on the colored pixels 588 a, 588 b and 588 c. Inaddition to being a reference electrode to the pixel electrode (notshown), the common electrode also protects the color filters fromcontacting the liquid crystal layer (not shown). Optionally, a furtherprotective layer 591 can overlie the common electrode 590. The furtherprotective layer can be a transparent insulator such as polyimide. Sucha protective layer may also serves as an alignment layer (see, e.g., 532in FIG. 24), which directs the polarization directions of the liquidcrystal through, for example, rubbing at about 250° C.

FIG. 26B shows a cross-sectional view of a color filter substrate in anIPS mode LCD according to another embodiment. The color filter substrate592 includes a transparent substrate 594 having a displaying surface595. A black matrix 596 is disposed on the transparent substrate 594opposite from the displaying surface 595. First, second and thirdcolored pixels 598 a, 598 b and 598 c for transmitting R, G and B colorlights, respectively are formed on at least one portion of thetransparent substrate 594 free of the black matrix 596. A protectivelayer 600 is formed on the first to third colored pixels 598 a, 598 band 598 c and protect them from contacting with the liquid crystal (notshown). The protective layer 600 is typically a transparent insulator,such as polyimide. A common electrode 602 based on conductive nanowiresis formed on the displaying surface 595 of the transparent substrate594. In the case of IPS display, the common electrode 602 serves as anelectrostatic discharge layer to avoid charging of the color filterplate.

The color filter substrate can be fabricated according to known methodsin the art. The optical transparence and the electrical conductivity ofthe nanowire-based transparent conductor film make it suitable as analternative electrode material in the color filter substrate of an LCD,regardless of the mode. The nanowire-based transparent conductor filmcan be coated or transferred from a laminated structure directly ontocommercial color filters according to the methods described herein. Atransparent conductive layer as described herein can also be used in acolor-on-array display structure or other LCD structures using atransparent electrode.

(b) Plasma Display Panel

A plasma display panel emits visible light by exciting fluorescentmaterials (e.g., phosphors) with ultraviolet light generated by a plasmadischarge. The plasma display panel employs two insulating substrates(e.g., glass plates), each insulating substrate having electrodes andbarrier ribs formed thereon to define individual cells (pixels). Thesecells are filled with one or more inert gases (e.g., Xe, Ne or Kr),which can be ionized under an electrical field to produce the plasma.More specifically, address electrodes are formed behind the cells, alonga rear glass plate. Transparent display electrodes, along with buselectrodes, are mounted in front of the cells, on a front glass plate.The address electrodes and the transparent display electrodes areorthogonal from one another and cross paths at the cells. In operation,a control circuitry charges the electrodes, creating a voltagedifference between the front and back plates and causing the inert gasesto ionize and form the plasma.

Metal oxide transparent conductors (e.g., ITO) are conventionally usedas the transparent display electrodes on the upper glass plate to allowthe plasma-generated visible light through. Current processing steps,notably, the sintering of the bus electrode and firing a frontdielectric layer, can require high processing temperatures (up to 400 to550° C.). For example, in order to obtain adequate transparency, thefront dielectric layer (which covers and protects the display electrodeand bus electrodes) is fired at about 570-580° C. At these temperatures,the resistivity of the ITO increases by about three-fold.

Nanowire-based transparent conductors are suitable electrode materialsfor the display electrodes in a PDP. They are demonstrated to beelectrically and optically stable at high temperatures (e.g., at 300°C.). They can be patterned at the desired features sizes (e.g., 100-300μm) for PDPs; in particular, they can be patterned by thehigh-throughput methods described herein.

FIG. 27 shows a cross-sectional view of a PDP according to oneembodiment. The PDP 606 includes: a lower transparent substrate 608; alower insulation layer 610 formed on the lower transparent substrate608; an address electrode 612 formed on the lower insulation layer 608;a lower dielectric layer 614 formed on the address electrode 612 and thelower insulation layer 610; isolation walls 616 defining a dischargingcell 618; black matrix layers 620 positioned on the isolation walls 616;a fluorescent layer 622 formed on the sides of the black matrix layer620 and the isolation wall 616 and on the lower insulation layer 608; anupper transparent substrate 624; a display electrode 626 formed on theupper transparent substrate 624 and positioned at a right angle inrelation to the address electrode 612; a bus electrode 628 formed on aportion of the display electrode 626; an upper dielectric layer 630formed on the bus electrode 628, the display electrode 626 and the uppertransparent substrate 624; and a protection layer (e.g., MgO) 632 formedon the upper dielectric layer 630. The display electrodes are formed byconductive nanowire films and patterned according to methods describedherein.

It should be understood that the nanowire-based transparent conductorfilms are suitable for any other configurations of PDP in whichtransparent electrodes are positioned on a display panel such that lightcan transmit with acceptable efficiency to create an image on thedisplay panel.

(c) Touch Screens

In a further embodiment, the transparent conductor described hereinforms part of a touch screen. A touch screen is an interactive inputdevice integrated onto an electronic display, which allows a user toinput instructions by touching the screen. A touch screen devicetypically comprises two opposing electrically conductive layersseparated by a spacer layer. The conductive layers are optically clearto allow light and image to transmit through. Currently available touchscreens typically employ metal oxide conductive layers (e.g., ITOfilms). As noted above, ITO films are costly to fabricate and may besusceptible to cracking if used on a flexible substrate. In particular,ITO films are typically deposited on glass substrates at hightemperature and in vacuo. In contrast, the transparent conductorsdescribed herein can be fabricated by high throughput methods and at lowtemperatures. They also allow for diverse substrates other than glass.For example, flexible and durable substrates such as plastic films canbe coated with nanowires and become surface-conductive.

Thus, FIG. 28 shows schematically a resistive touch screen device 640according to one embodiment. The device 640 includes a bottom panel 642comprising a first substrate 644 coated or laminated with a firstconductive layer 646, which has a top conductive surface 648. An upperpanel 650 is positioned opposite from the bottom panel 642 and separatedtherefrom by adhesive enclosures 652 and 652′ at respective ends of thedevice 640. The upper panel 650 includes a second conductive layer 654coated or laminated on a second substrate 656. The second conductivelayer 654 has an inner conductive surface 658 facing the top conductivesurface 648 and is suspended over spacers 660.

When a user touches the upper panel 650, the inner conductive surface658 and the top conductive surface 648 of the bottom panel 642 come intoelectrical contact. A contact resistance is created, which causes achange in the electrostatic field. A controller (not shown) senses thechange and resolves the actual touch coordinate, which information isthen passed to an operating system.

According to this embodiment, either or both first and second conductivelayers are based on conductive nanowire layers, as described herein. Theinner conductive surface 658 and the top conductive surface 648 eachhave surface resistivity in the range of about 10-1000 Ω/□, morepreferably, about 10-500 Ω/□. Optically, the upper and bottom panelshave high transmission (e.g., >85%) to allow for images to transmitthrough.

In certain embodiments, the transparent conductive layer can be furthercoated with a protective layer (e.g., a dielectric overcoat), whichimproves the durability of the transparent conductive layer. However,making electrical contact with the underlying metal nanowires may becomeproblematic because contact resistance cannot be reliably created due tothe intervening dielectric overcoat(s). In addition, even slightvariations in thickness in the overcoat may result in non-contact pointson the film. Thus, in these embodiments, the overcoat layer isincorporated with conductive particles to create reliable electricalcontacts and to improve contact resistance.

FIG. 28A schematically shows two opposing conductive layers withrespective overcoats. More specifically, the first conductive layer 646is coated with a first overcoat 647 and the second conductive layer 654is coated with a second overcoat 655. The first and second overcoats areembedded with conductive particles 657. The presence of the conductiveparticles in the dielectric overcoats increases their surfaceconductivity and provides electrical connection between the underlyingnanowire-filled conductive layers.

Compared to the respective underlying nanowire-filled transparentconductive layer, the overcoat can tolerate much higher resistivity.Unlike the nanowires in the underlying conductive layer 646 or 654,which must form a conductive network above the electrical percolationthreshold to ensure an in-plane conductivity (e.g., between 10-1000 Ω/□in resistivity), the conductive particles in the overcoat do not need toreach the electrical percolation threshold. For example, the sheetresistance of the overcoat can be as high as 10⁸ Ω/□. Even at thislevel, the resistivity through the overcoat is low enough for touchscreen applications.

The overcoat can be formed of any of the optically clear polymericmatrix materials described herein. The thickness of the overcoat istypically less than 2 μm or less than 1 μm. Typically, a thickerovercoat is likely to result in a higher contact resistance.

Any type of nano-sized conductive particles can be used. Examples ofsuitable conductive particles include, but are not limited to, ITO, ZnO,doped ZnO, metallic nanowires, metallic nanotubes or carbon nanotubes(CNT) as described herein. The sizes of the conductive particles aretypically lower than 200 nm to maintain an acceptable level of haze.More typically, they are lower than 100 nm. Because the loading level ofthe conductive particles is so low, their presence typically does notaffect the optical transmission. On the other hand, the presence of theconductive particles may provide a certain degree of surface roughnessthat serves to reduce glare.

In certain embodiments, the conductive particles can be a mixture ofhighly conductive particles (e.g., metal nanowires) and low-conductivityparticles (e.g., ITO or ZnO powders). While the metal nanowires do nothave to form a conductive network within the matrix (i.e., above theelectrical percolation threshold), they provide a high-conductivity pathover a relatively large distance. The current will be mostly transportedin these nanowires while the low-conductivity particles will provide theelectrical connection between the nanowires. Advantageously, the sheetresistance of the overcoat can be controlled in a wider range byadjusting the ratio of the nanowires to low-conductive particles. Sincethe nanowires do not have to form a percolative network, it is expectedthat the resistivity of the final overcoat film will be in a more linearrelationship with the nanowire concentration and stable at higher sheetresistances than using the low-conductivity powders alone. The mixtureof the metal nanowires and conductive powders can be co-deposited with amatrix material in a one-pass process. Alternatively, in a two-passprocess, a nanowire layer can be deposited (without necessarily forminga percolative network) prior to depositing the overcoat layer embeddedwith the low-conductive particles.

The first and second substrates can be a variety of materials, asdescribed herein. For example, the first substrate can be rigid (e.g.,glass, or rigid plastic such as polycarbonate or polyacrylate) while thesecond substrate can be a flexible film. Alternatively, for a flexibletouch screen application, both substrates can be flexible films (e.g.,plastic).

As known in the art, touch screen devices may also be made includingonly a single substrate having a transparent conductor and both thistype of touch screen device and the two conductor type described abovemay include a third transparent conductor that functions as anelectrostatic discharge layer. The transparent conductor describedherein may be used in any of these types of touch panel devices.Additionally, nanowire-based transparent conductors used in such devicesmay be patterned as described herein or any other way known in the art.

(d) Photovoltaic Cells

Solar radiation provides a usable energy in the photon range ofapproximately 0.4 eV to 4 eV. Optoelectronic devices such asphotovoltaic (PV) cells can harvest and convert certain photon energiesin this range to electrical power. Photovoltaic cells are essentiallysemiconductor junctions under illumination. Light is absorbed by thesemiconductor junction (or diode) and electron-hole pairs are generatedon both sides of the junction, i.e., in the n-type emitter and in thep-type base. These charge carriers—electrons from the base and holesfrom the emitter—then diffuse to the junction and are swept away by theelectric field, thus producing electric current across the device.

The semiconductor junction can be formed in a homojunction cell bydoping a single material (e.g., crystalline silicon) to form p-type andn-type sides. Either PN structure or P-i-N structure can be used.

A heterojunction can be formed by contacting two differentsemiconductors. Typically, the two semiconductors have different bandgaps. The one with the higher bandgap is selected for its transparencyand positioned as a top layer or window layer. The one with the lowerbandgap forms a bottom layer, which functions as a light-absorbingmaterial. The window layer allows almost all incident light to reach thebottom layer, which readily absorbs light.

Multi-junction cells have been developed to capture a larger portion ofthe solar spectrum. In this configuration, individual heterojunctioncells are stacked in such a way that sunlight falls first on thematerial having the largest bandgap. Photons not absorbed in the firstcell are transmitted to the second cell, which then absorbs thehigher-energy portion of the remaining solar radiation while remainingtransparent to the lower-energy photons. These selective absorptionprocesses continue through to the final cell, which has the smallestbandgap.

In excitonic PV cells, instead of a p-doped and n-doped region, thematerials of different band gaps are used to split and exciton viacharge transfer from one semiconductor to the other. After chargeseparation, the charges are swept away due to built in potential whichis created due to the difference in work function between the contactelectrodes for charge collection. Organic photovoltaic cells, forexample, work this way where one semiconductor can be a polythiopheneand the other C60. Polythiophene absorbs light and an exciton iscreated. The electron jumps from polythiophene to C60 (lower energystate for the electron). The holes move along the polytiophene backboneuntil they are collected as do the electrons by hopping betweenbuckyballs.

Ohmic metal-semiconductor contacts are provided to both the n-type andp-type sides of the solar cell. In multi-junction cells, they are alsointerposed between two adjacent cells. Electrons that are created on then-type side, or have been “collected” by the junction and swept onto then-type side, may travel through the wire, power the load, and continuethrough the wire until they reach the p-type semiconductor-metalcontact.

Because transparent conductors (e.g., ITO) allow light to pass through awindow layer to the active light absorbing material beneath, as well asserve as ohmic contacts to transport photo-generated charge carriersaway from that light absorbing material, they are desired as contactmaterials for solar cells.

Nanowire-based transparent conductor films described herein can be usedas one or more contacts in a solar cell. Unlike the conventional metaloxide transparent conductor, nanowire-based transparent conductor can beformed on a flexible substrate by high through-put methods. They also donot require the special deposition (in vacuum) that makes the use ofmetal oxide layers costly in solar cell applications.

Thus, one embodiment describes a homojunction solar cell comprising atop contact, a semiconductor diode, and a bottom contact, wherein one orboth of the top contact and bottom contact can be made of nanowire-basedtransparent conductor film. FIG. 29A shows a homojunction solar cell664. The solar cell 644 includes a top contact 668, a bottom contact670, and a semiconductor diode 672 interposed therebetween.

The semiconductor diode can be, for example, a PN structure with thep-doped silicon on the top and N-doped silicon on the bottom. Thesilicon is typically crystalline silicon. As a more economicalalternative, polycrystalline silicon can be used according to knownmethods in the art. The semiconductor diode can also be formed ofamorphous silicon, in which case, a P-i-N structure is preferred.

The top contact and the bottom contact can be prepared by the methodsdescribed herein. The top contact is typically optically clear andcomprises a light incidence surface, i.e., the surface that lightinitially enters in the solar cell.

Optionally, a substrate 674 can be present underneath the bottom contact670. Also optionally, bus bars 676 can be formed overlying the topcontact. The bus bars 676 can also be formed by patterned nanowire-basedtransparent conductor films.

FIG. 29B shows a heterojunction solar cell according to anotherembodiment. As shown, the heterojunction solar cell 680 includes a topcontact 682, a bottom contact 684, and a semiconductor heterojunctionlayer 686 interposed therebetween. One or both of the top contact 682and bottom contact 684 can be made of nanowire-based transparentconductor film.

In certain embodiments, the semiconductor heterojunction layer 686comprises a three-layer structure (e.g., N-i-P). Thus, it may comprise adoped top semiconductor layer 686 a, an undoped middle semiconductorlayer 686 b and a doped bottom semiconductor layer 686 c. In certainembodiment, the first semiconductor layer 686 a has a higher bandgapthan the third semiconductor layer 686 c.

The first, second and third semiconductor layers can be deposited asthin film layers. Suitable semiconductor materials include, but are notlimited to, organic semiconductor materials (as discussed herein),cadmium telluride (CdTe), copper indium gallium selenide (CIGS), copperindium selenide (CIS), and the like. For example, in a typical CdTecell, the top layer is p-type cadmium sulfide (CdS), the middle layer isintrinsic CdTe, and the bottom layer is n-type zinc telluride (ZnTe).

It is also possible for the semiconductor heterojunction layer 686 toinclude only the top semiconductor layer 686 a and the bottomsemiconductor layer 686 c in a NP structure.

Heterojunction cells based on thin film semiconductor layers savematerial cost, compared to that of silicon-based solar cells. However,due to inferior performance of thin film semiconductor layers, suchdevices are less efficient than the poly-silicon-based cells in energyconversion. Thus, in one embodiment, a multijunction cell is describedin connection with FIG. 29C. As shown, the multijunction cell 690includes, sequentially from top to bottom: a top contact 692, a firstcell 694, a tunnel layer 696, a second cell 698 and a bottom contact700, wherein the top contact 692 and the bottom contact 700 are made ofnanowire-based transparent conductor films. For purpose of simplicity,the multijunction cell 690 is shown to include only two cells. It shouldbe understood, however, that additional cells can be fabricated insimilar fashion.

Both the first cell 692 and the second cell 698 have similar 3-layerstructures as the single junction solar cell 680 shown in FIG. 29B. Thefirst cell is closer to an impinging light and should therefore beselected to have a larger bandgap than that of the second cell. In doingso, the first cell is transparent to the lower energy photons, which canbe absorbed by the second cell 698.

The first and second cells are separated by a tunnel layer 696 to allowthe flow of electrons between the cells. The tunnel layer 696 can be aPN diode comprising oppositely doped semiconductor layers.

The top contacts 692 and the bottom contacts 700 are nanowire-basedtransparent conductor films. They can be prepared by the methodsdescribed herein.

The solar cell 690 can include additional layers such as substrates, busbars, anti-reflective films and the like, as will be recognized by oneskilled in the art. Further, it should be understood that thenanowire-based transparent conductor films are suitable as one or morecontacts in any solar cell configurations.

(e) Electroluminescent Devices.

Electroluminescence (EL) is the result of radiative recombination ofelectrons and holes in a material (usually a semiconductor). The excitedelectrons release their energy as photons as they return to their groundstate. Prior to recombination, electrons and holes are separated eitheras a result of doping of the material to form a p-n junction (insemiconductor electroluminescent devices such as LEDs), or throughexcitation by impact of high-energy electrons accelerated by a strongelectric field (as with the phosphors in electroluminescent displays).

EL devices are useful as, for example, backlighting for displays. Theytypically comprise two electrodes sandwiching an EL material, with atleast one of the electrodes being transparent. Thin film designs of ELdevices are possible by employing thin conductive films. Advantageously,patterned lighting can be achieved by using a patterned transparentconductor as one of the electrodes. Conventionally, polymeric conductivefilms such as poly(dioxythiophene) (PDOT) are used and patterned as thetransparent conductor.

The nanowire-based transparent conductor films described herein aresuitable as a transparent electrode in an EL device. Thus, FIG. 30 showsa thin film EL device according to one embodiment. The EL device 710comprises, sequentially from the top: a top electrode 712, a dielectriclayer 714, an EL material layer 716, a transparent electrode 718, anoptional barrier layer 720, and an optional overcoat 722.

The transparent electrode 718 is formed by a nanowire layer, asdescribed herein. The optical transmission and surface conductivity of ananowire layer make it suitable as the transparent electrode. Moreover,as discussed in more detail in Example 8, the nanowires can beformulated into a screen printable ink composition. The ink compositioncan be printed to provide patterned electrodes, which is compatible withthe current manufacturing process for patterning PDOT layer. Otherpatterning methods can also be used, including stamping, slot-die, andthe like.

The EL material layer 716 can be any acceptable EL material, forexample, phosphor. The optional barrier layer 720 can be a fluoropolymer(e.g., Kyner) to keep out adverse environmental factors, such asmoisture.

(f) Electrostatic Dissipative Material—Anti-Static Coating

Electrostatic discharge (ESD) is a single-event, rapid transfer ofelectrostatic charge between two objects, usually resulting when twoobjects at different potentials come into direct contact with eachother. Electrostatic charge build-up occurs as a result of an imbalanceof electrons on the surface of a material. ESD is one of the majorcauses of device failures in the semiconductor industry.

Electrically conductive materials (e.g., conductive films or coatings)are effective as electrostatic dissipative materials by channelingharmful electrostatic charge away. Typically, a conductive layer havinga surface resistivity of no more than 10⁸ Ω/□ is effective in mitigatingor eliminating electrostatic charge build-up. More typically, ananti-static coating has a surface resistivity of between about 10⁶ Ω/□and 10⁷ Ω/□.

Accordingly, nanowire-based conductive layers described herein aresuitable as anti-static coatings. Any of the structures described in,for example, FIGS. 10B-10F, can be used as anti-static coating on asubstrate in need thereof. Thus, one embodiment provides a method forproviding electromagnetic shielding comprising; providing a compositeincluding a plurality of metallic nanowires and a matrix material;applying the composite to a substrate in need of electromagneticshielding; and forming a conductive layer including the plurality ofmetallic nanowires dispersed in the matrix material, the conductivelayer having a surface conductivity of no more than 10⁸ Ω/□.

In certain embodiment, the anti-static coating is optically clear suchthat the underlying substrate is visible. In another embodiment, anoptically clear substrate (e.g., plastic) coated with nanowire-basedconductive layer can be used as a packaging material for electronics.The optical transparency allows for a direct visualization of thecontent in the package.

The transparent conductor structures, their electrical and opticalproperties, and the methods of fabrication are illustrated in moredetail by the following non-limiting examples.

EXAMPLES Example 1 Synthesis of Silver Nanowires

Silver nanowires were synthesized by the reduction of silver nitratedissolved in ethylene glycol in the presence of poly(vinyl pyrrolidone)(PVP) following the “polyol” method described in, e.g. Y. Sun, B. Gates,B. Mayers, & Y. Xia, “Crystalline silver nanowires by soft solutionprocessing”, Nanoletters, (2002), 2(2) 165-168. A modified polyolmethod, described in U.S. application Ser. No. 11/766,552, in the nameof Cambrios Technologies Corporation, produces more uniform silvernanowires at higher yields than does the conventional “polyol” method.This application is incorporated by reference herein in its entirety.

Example 2 Preparation of a Transparent Conductor

An Autoflex EBG5 polyethylene terephthalate (PET) film 5 μm thick wasused as a substrate. The PET substrate is an optically clear insulator.The light transmission and haze of the PET substrate are shown in Table2. Unless specified otherwise, the light transmission was measured usingthe methodology in ASTM D1003.

An aqueous dispersion of silver nanowires was first prepared. The silvernanowires were about 70 nm to 80 nm in width and around 8 μm in length.The concentration of the silver nanowires (AgNW) was about 0.5% w/v ofthe dispersion, resulting in an optical density of about 0.5 (measuredon a Molecular Devices Spectra Max M2 plate reader). The dispersion wasthen coated on the PET substrate by allowing the nanowires to sedimentonto the substrate. As understood by one skilled in the art, othercoating techniques can be employed e.g., flow metered by a narrowchannel, die flow, flow on an incline and the like. It is furtherunderstood that the viscosity and shear behavior of the fluid as well asthe interactions between the nanowires may affect the distribution andinterconnectivity of the nanowires coated.

Thereafter, the coated layer of silver nanowires was allowed to dry bywater evaporation. A bare silver nanowire film, also referred to as a“network layer”, was formed on the PET substrate. (AgNW/PET) The lighttransmission and haze were measured using a BYK Gardner Haze-gard Plus.The surface resistivity was measured using a Fluke 175 True RMSMultimeter. The results are shown in Table 2. The interconnectivity ofthe nanowires and an areal coverage of the substrate can also beobserved under an optical or scanning electron microscope.

The matrix material was prepared by mixing polyurethane (PU) (MinwaxFast-Drying Polyurethane) in methyl ethyl ketone (MEK) to form a 1:4(v/v) viscous solution. The matrix material was coated on the baresilver nanowire film by spin-coating. Other known methods in the art,for example, doctor blade, Meyer rod, draw-down or curtain coating, canbe used. The matrix material was cured for about 3 hours at roomtemperature, during which the solvent MEK evaporated and the matrixmaterial hardened. Alternatively, the curing can take place in an oven,e.g., at a temperature of 50° C. for about 2 hours.

A transparent conductor having a conductive layer on the PET substrate(AgNW/PU/PET) was thus formed. The conductive layer of the silvernanowires in the matrix was about 100 nm thick. Its optical andelectrical properties were measured and the results are shown in Table2.

The transparent conductor was further subjected to a tape test. Morespecifically, a 3M Scotch® 600 adhesive tape was firmly applied to thesurface of the matrix and then removed, for example, by peeling. Anyloose silver nanowires were removed along with the tape. After the tapetest, the optical and electrical properties of the transparent conductorwere measured and the results are shown in Table 2.

By way of comparison, a matrix-only film was formed on a PET substrate(PU/PET) under the same conditions as described above. The opticalproperties (light transmission and haze) and the electrical propertiesof the PU/PET are also provided in Table 2.

As shown in Table 2, the matrix-only film on PET (PU/PET) had a slightlyhigher light transmission as well as haze value than a PET substrate.Neither was conductive. By comparison, the bare silver nanowire film onPET was highly conductive, registering a surface resistivity of 60 Ω/□.The deposition of the bare silver nanowire film on the PET lowered thelight transmission and increased the haze. However, the bare silvernanowire film on PET was still considered optically clear with a lighttransmission of more than 80%. The optical and electrical properties ofthe bare silver nanowire film on PET were comparable or superior tometal oxide films (e.g., ITO) formed on PET substrates, which typicallyrange from 60 to 400 Ω/□.

As further shown in Table 2, the transparent conductor based on silvernanowires in the polyurethane matrix had an almost identical lighttransmission as the bare silver nanowire film on PET, and a slightlyhigher haze. The resistivity of the transparent conductor remained thesame as the bare silver nanowire film, indicating that the coating ofthe matrix material did not disturb the silver nanowire film. Thetransparent conductor thus formed was optically clear, and had acomparable or superior surface resistivity to metal oxide films (e.g.,ITO) formed on PET substrates.

In addition, the tape test did not alter the resistivity or the lighttransmission of the transparent conductor, and only slightly increasedthe haze.

TABLE 2 Light Transmission Transparent Media (%) Haze (%) Resistivity(Ω/□) PET 91.6 0.78 non-conductive PU/PET 92.3 0.88 non-conductiveAgNW/PET 87.4 4.76 60 AgNW/PU/PET 87.2 5.74 60 After tape test 87.2 5.9460

Example 3 Accelerated H₂S Corrosion Tests

Sulfides, such as hydrogen sulfide (H₂S), are known corrosive agents.The electrical properties of the metal nanowires (e.g., silver) canpotentially be affected in the presence of the atmospheric sulfides.Advantageously, the matrix of the transparent conductor serves as a gaspermeation barrier. This prevents, to certain degree, the atmosphericH₂S from contacting the metal nanowires embedded in the matrix.Long-term stability of the metal nanowires can be further obtained byincorporating one or more corrosion inhibitors in the matrix, asdescribed herein.

In the United States, the amount of H₂S in the air is about 0.11-0.33parts per billion (ppb). At this level, corrosion is expected to takeplace over an extended period of time. Thus, an accelerated H₂Scorrosion test was designed to provide an extreme case of H₂S corrosion.

Freshly cooked egg yolks were broken into pieces and sealed in a plasticbag. A H₂S meter (Industrial Scientific, GasBadge Plus—Hydrogen SulfideSingle Gas Monitor) was inserted into the bag to monitor the release ofH₂S from the egg yolks. FIG. 31 shows a typical release profile of theH₂S gas over a period of 24 hours. After an initial build-up of the H₂Sin the bag, the gas level dropped, indicating that the gas had diffusedout of the permeable bag. Nevertheless, the levels of the H₂S gas(peaked at 7.6 ppm) in the bag greatly surpassed the level of theatmospheric H₂S gas.

A bare silver nanowire film on PET was prepared according to Example 2.The film was placed in a plastic bag with freshly cooked egg yolks. Thefilm began to darken within two hours, indicating that the silver hadbeen tarnished and black Ag₂S was formed. In contrast, color changes infilms of silver nanowires in polyurethane matrix were not observed untilafter 2-3 days, indicating that the polyurethane matrix acted as abarrier to slow down the permeation of the H₂S gas.

Example 4 Incorporation of Corrosion Inhibitors

The following samples of conductive films were prepared. A PET substratewas used for each sample. In certain samples, corrosion inhibitors,including benzotriazole, dithiothiadiazole and acrolein, wereincorporated during the preparation of the conductive films.

Samples 1-2 were prepared according to the method described herein. Nocorrosion inhibitor was present.

Sample 1 was a conductive film of bare silver nanowires.

Sample 2 was a conductive film of silver nanowires in a polyurethanematrix.

Samples 3-6 were prepared by first forming a bare silver nanowire filmon a PET substrate (i.e., Sample 1). Thereafter, various corrosioninhibitors were incorporated during the coating processes of the matrixmaterial.

Sample 3 was prepared by coating a 0.1 w/v % solution of benzotriazole(BTA) in methyl ethyl ketone (MEK) on the bare silver nanowire film,allowing the solvent to dry after coating, followed by coating thematrix material of polyurethane (PU) in MEK (1:4).

Sample 4 was prepared by first incorporating 1.5 v/v % ofdithiothiadiazole in a matrix material PU/MEK (1:4), followed by coatingthe matrix material on the bare silver nanowire film.

Sample 5 was prepared by first dipping a bare silver nanowire film in asolution of 1.5 v/v % of dithiothiadiazole in MEK, allowing the solventto dry after dipping, followed by coating with a matrix material PU/MEK(1:4) having 1.5 v/v % of dithiothiadiazole.

Sample 6 was prepared by first incorporating 1.5 v/v % of acrolein in amatrix material PU/MEK (1:4), followed by coating the matrix material ona bare silver nanowire film.

The optical and electrical properties of the Samples 1-6 were measuredbefore and after an accelerated H₂S treatment, as described in Example3. The results are shown in FIGS. 32A, 32B and 32C.

FIG. 32A shows the light transmission measurements of Samples 1-6 beforethe H₂S treatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the decrease of light transmission for each sample is alsographed. Prior to the H₂S treatment, all of the samples were shown to beoptically clear (having a light transmission higher than 80%). Following24 hours of the H₂S treatment, all of the samples have experienceddecreases in their light transmissions due to different degrees ofsilver tarnish.

As expected, Sample 1 had the most reduction in light transmission.Samples 3 and 6 did not perform better than the matrix-only sample(Sample 2). Samples 4 and 5, however, had less reduction in lighttransmission compared to the matrix-only sample, indicating thecorrosion inhibitor dithiothiadiazole was effective in protecting thesilver nanowires from being corroded.

FIG. 32B shows the resistance measurements of Samples 1-6 before the H₂Streatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the decrease of the resistance for each sample is alsographed. As shown, all but Sample 4 experienced dramatic increases intheir resistances and effectively became non-conductive, although theonset of the degradation in electrical properties was significantlydelayed for some samples. Sample 4 had only a modest increase in itsresistance. It is noted that the impacts of H₂S on Sample 4 and Sample 5differed considerably, despite that both Samples had the same corrosioninhibitor (dithiothiadiazole). This implies that the coating processesmay affect the effectiveness of a given corrosion inhibitor.

FIG. 32C shows the haze measurements of Samples 1-6 before the H₂Streatment and 24 hours after the H₂S treatment. For purpose ofcomparison, the change in the haze for each sample is also graphed. Allthe samples showed increases in their haze measurements. With theexception of Samples 1 and 6, the haze was within acceptable range (lessthan 10%) for each of Samples 2-5.

Sample 4 was shown to have the best overall performance in withstandingthe corrosive H₂S gas. By incorporating the corrosion inhibitor(dithiothiadiazole) in the matrix, the transparent conductor showedclear advantage over Sample 2 in which no corrosion inhibitor waspresent.

It is noted that the H₂S levels in these accelerated tests were fargreater than the atmospheric H₂S. It is therefore expected thattransparent conductors prepared similarly as Sample 4 would fare evenbetter in the presence of the atmospheric H₂S.

Example 5 Pressure-Treatment of Metal Nanowire Network Layers

Table 3 illustrates the results of two trials of applying pressure to asurface of a silver nanowire network layer (or “network layer”) on asubstrate.

Specifically, silver nanowires of around 70 nm to 80 nm in width andaround 8 μm in length were deposited on an Autoflex EBG5 PET substrate.The substrate was treated with Argon plasma prior to the deposition ofthe nanowires. A network layer was formed according to the methoddescribed in Example 2. No matrix material was applied to the networksprior to the pressure treatment. The Trials listed in Table 2 werecarried out using a single stainless steel roller on a rigid bench-topsurface. The area of the network layer treated was from 3 to 4 incheswide and from 3 to 4 inches long.

TABLE 3 Trial 1 Trial 2 Transmission Process R (Ω/square) R (Ω/square)(%) Haze (%) (original) 16000 400000 88.2 3.34 1 roll @ 340 psi 297 69087.3 3.67 1 roll @ 340 psi 108 230 87.2 4.13 1 roll @ 340 psi 73 12786.6 4.19 1 roll @ 340 psi 61 92 87.1 4.47 1 roll @ 340 psi 53 86 86.64.44 Ar plasma 38 62 88.0 4.19

Prior to any application of pressure, the network layers had theresistance indicated in the “original” row (the network layers were notpre-treated with plasma.) Each row of Table 3 indicates a subsequentsingle roll across the network layer at approximately 340 psi.

In each trial, the network layer was rolled 5 times. Thereafter, aplasma treatment was applied to the network layer. The resistance aftereach roll is as listed in the second (first trial) and third (secondtrial) columns. Variation in transmission and haze for the second trialis as listed in the fourth and fifth columns, respectively. As shown, itwas determined that the conductivity of the network layer of each trialwas increased by application of pressure to a surface thereof.

As shown above in Table 3, application of pressure to a network layer bya roller can reduce the light transmission of the layer and increase thehaze. As shown in Table 4 below, a washing process following thepressure treatment can further improve the transmission and reduce thehaze of the network layer.

TABLE 4 Process Resistance (Ω/□) Transmission (%) Haze (%) (original)700,000 86.4 4.77 2 rolls @ 340 psi 109 85.6 5.24 soap & water wash 4486.0 4.94 Ar plasma 24 85.9 4.81

As shown in Table 4, application of pressure to a network layer by twicerolling with a single stainless steel bar at approximately 340 psi on arigid surface reduced the light transmission and increased the haze ofthe network layer. Washing the network layer with soap and water afterthe rolling, however, increased the transmission and decreased the haze.An argon plasma treatment further improved the transmission and haze.

Washing the network with soap and water without rolling is alsoeffective at improving the conductivity to some extent.

Following the pressure or washing treatments, a matrix material can becoated as previously described in Example 2.

Example 6 Photo-Patterning of Conductive Layers

FIG. 33 illustrates one method of directly patterning a nanowire-basedtransparent conductive film. In this example, a silver nanowire networklayer (“network layer”) 726 was initially formed on a glass substrate728 according to the method described in Example 2. Two holders 730 wereplaced on the glass substrate 726 to define an area 732 for matrixformation. A photo-curable matrix material 734 comprising a mixture ofprepolymers was coated over the network layer 726 within the area 732. Amask 736 was placed upon the holders 730. The mask 736 was a glass slidehaving a array of dark lines 738 of about 500 μm wide. The matrixmaterial was then irradiated under a Dymax 5000 lamp for 90 seconds. Thematrix material cured in the regions exposed to light and remainedliquid in the regions that were masked by the dark lines.

FIGS. 34A-F show images of a photo-patterned conductive layer under anoptical microscope. FIG. 34A shows a conductive film 740 immediatelyafter photo-curing (5×). The lighter region 748 was exposed to UVirradiation and cured. The darker region 744 was masked from the lightexposure and the matrix material therein was uncured. Conductive film740 was further subjected to an adhesive tape or a tacky roll treatmentto remove the uncured matrix material and the nanowires in the uncuredregion 744. FIG. 34B shows the conductive film 740 after the adhesivetape treatment (5×), in which the cured region 748 appears much lighterthan the uncured region 744. At higher magnification (FIGS. 34C and 34D,20×), it can be observed that the uncured region 744 has a lowerconcentration of nanowires than the cured region 748. This contrast ismore apparent in FIGS. 34E and 34F (100×). It was further observed that,following the adhesive tape treatment, the concentration of thenanowires dropped below the percolation threshold in the uncured region744. Electrical measurements using fine probe tips showed that theuncured region 744 was non-conductive.

As an alternative to removing the matrix material and nanowires in theuncured region using adhesive tapes or tacky rolls, a solvent may beused to wash the uncured regions. As shown in FIGS. 35A-D, a conductivefilm 750 was prepared as described above and exposed to UV irradiationthrough a brass aperture mask. FIG. 35A shows cured regions (conductiveregions) 752 and uncured regions 754 after being washed with ethanol andwiped. FIGS. 35B-D illustrate, at increasing magnifications, thecontrast of the nanowire concentration in the uncured regions 754compared to that in the cured regions 752. In the uncured regions 754,most of the uncured matrix material and the silver nanowires had beenremoved by the ethanol washing. Photo-patterning therefore producesconductive regions and non-conductive region according to apredetermined pattern.

Example 7 Photo-Curable Formulations

The matrix material described in Example 6 can be formulated bycombining an acrylate monomer (or prepolymer, as defined herein), amulti-functional acrylate monomer (or prepolymer) and at least onephotoinitiator. Any acrylate monomers or prepolymers can be used, suchas epoxy acrylates, more specifically, 2-ethylhexyl acrylate,2-phenoxyethyl acrylate, lauryl acrylate, methacrylates, and the like.Any multi-functional acrylate monomer (or prepolymer) can be used topromote the formation of a crosslinking polymer network. Examplesinclude trimethylolpropane triacrylate (TMPTA), tripropylene glycoldiacrylate, bisphenol-A diacrylate, propoxylated (3) trimethylolpropanetriacrylate, dipentaerythritol penta-acrylate. Any photoinitiator, forexample, ketone based initiators, can be used. Specific examplesinclude: Ciba Irgacure 754, phenyl ketone such as Ciba Irgacure 184,α-hydroxy ketones, glyoxylates, benzophenone, α-amino ketones and thelike. More specifically, a fast-curing formulation can be prepared bycombining 60%-70% 2-ethylhexyl acrylate, 15%-30% trimethylolpropanetriacrylate and about 5% Ciba Irgacure 754.

Other additives can be added to enhance the stability and/or promote theadhesion of the matrix and the nanowires. For example, an adhesionpromoter (e.g., silanes) that promotes the coupling between organicmatter and inorganic matter can be used. Examples of the silane-typeadhesion promoters include GE Silquest A174, GE Silquest A1100 and thelike. Antioxidants such as Ciba Irgonox 1010ff, Ciba Irgonox 245,Irgonox 1035 can be used. Moreover, additional or co-initiators can beused to promote the efficiency of the photoinitiator. Examples ofcoinitiator can include any types of tertiary amine acrylates, such asSartomer CN373, CN 371, CN384, CN386 and the like. An additionalphotoinitiator such as Ciba Irgacure OXE01 can be further added.

Below are four exemplary photo-curable formulations suitable as thematrix material used in this example:

Formulation 1

75% 2-ethylhexyl acrylate;

20% trimethylolpropane triacrylate (TMPTA);

1% adhesion promoter (GE Silquest A1100);

0.1% antioxidant (Ciba Irgonox 1010ff) and

4% photoinitiator (Ciba Irgacure 754)

Formulation 2

73.9% 2-ethylhexyl acrylate;

20% trimethylolpropane triacrylate (TMPTA);

1% adhesion promoter (GE Silquest A1100);

0.05% antioxidant (Ciba Irgonox 1010ff) and

5% photoinitiator (Ciba Irgacure 754)

Formulation 3

73.1% tripropylene glycol diacrylate (TPGDA)

22.0% trimethylolpropane triacrylate (TMPTA)

4.9% photoinitiator (Ciba Irgacure 754)

0.03% antioxidant (4-methoxyphenol)

Formulation 4

68% 2-ethylhexyl acrylate;

20% trimethylolpropane triacrylate (TMPTA);

1% adhesion promoter (GE Silquest A1100);

0.1% antioxidant (Ciba Irgonox 1010ff) and

5% photoinitiator I (Ciba Irgacure 754)

5% coinitiator (Sartomer CN373)

1% photoinitiator II (Ciba Irgacure OXE01)

Example 8 Nanowire Dispersion

A nanowire dispersion, or ink, was formulated by combining about 0.08%wt. HPMC, about 0.36% wt. silver nanowires, about 0.005% wt. Zonyl®FSO-100 and about 99.555% wt. water. As an initial step, an HPMC stocksolution was prepared. An amount of water equal to about ⅜ of the totaldesired volume of nanowire dispersion was placed in a beaker and heatedto between 80° C. and 85° C. on a hotplate. Enough HPMC to make 0.5% wt.HPMC solution was added to the water and the hotplate was turned off.The HPMC and water mixture was stirred to disperse the HPMC. Theremainder of the total amount of water was chilled on ice and then addedto the heated HPMC solution and stirred at high RPM for about 20 min.The HPMC solution was filtered through a 40 μm/70 μm (absolute/nominal)Cuno Betapure filter to remove undisolved gels and particulates. Next astock solution of Zonyl® FSO-100 was prepared. More specifically, 10 gof Zonyl® FSO 100 were added to 92.61 mL of water and heated until theZonyl® FSO 100 was fully dissolved. The necessary amount of HPMC stocksolution to make about 0.08% wt. HPMC solution in the final inkcomposition was placed in a container. Then, the necessary amount of DIwater to make about 99.555% wt. water solution in the final inkcomposition was added. The solution was stirred for about 15 min. andthe necessary amount of silver nanowires to make about 0.36% Ag nanowiresolution in the final ink composition were added. Finally, the necessaryamount of the Zonyl® FSO-100 stock solution to make about 0.005% wt.Zonyl® FSO-100 solution was added.

Example 9 Acid-Etching (1)

FIGS. 36A-36C show the progression of etching and the final patternformed on a transparent conductor sheet 758. More specifically, aconductive silver nanowires layer was first formed on a PET substrate. AUV-curable acrylate was deposited on the nanowire layer according to apattern. The matrix was allowed to dry and partially cured. The matrixwas typically about 50 nm-300 nm thick. Surface conductivity wasdetected in the areas protected by the matrix as well as in the areasunprotected by the matrix.

The transparent conductor sheet was then exposed to an acid-etchingsolution, which included 1% HNO₃, 1% NaNO₃ and 5 ppm of KMnO₄.

FIG. 36A shows that within one minute, the etching of the silvernanowires had begun from a region adjacent to an area protected by thematrix 760. The dissolution and disruption of the nanowire layer wasevident in the area unprotected by the matrix 762.

FIG. 36B shows the transparent conductor sheet two minutes into theetching. More unprotected silver nanowires were dissolved and awell-defined pattern emerged. FIG. 36C shows that at the end of fourminutes, all the unprotected silver nanowires had been etched andremoved by rinsing the transparent conductor sheet with water. Theprotected area 760 remain conductive. Optionally, the partially curedmatrix in the matrix protected area 760 can be further cured.

Example 10 Acid-Etching (2)

FIGS. 37A and 37B show the impact to the etching rate by using a higherconcentration of KMnO₄ in the acid-etching solution. A transparentconductor sheet 764 was prepared and patterned as described in Example9, except that 10 ppm of KMnO₄ was used. FIG. 37A shows that theunprotected nanowires were etched away within 30 seconds of the etching.FIG. 37B shows, in higher magnification, the well-defined pattern afteretching for about one minute. FIG. 37B also shows that the area 766 inwhich the matrix was present was not etched or disturbed by the etchingsolution. Like in Example 9, the protected area remainedsurface-conductive after the etching. Additionally, FIG. 37B shows thatat the interface between the conductive region and non-conductive regionof the patterned surface, nanowires are actually severed and portions ofthese severed nanowires that, prior to etching, extended into thenon-conductive region, are etched away. In this way, remaining portionsof the severed wires are shorter than the original lengths of the wiresprior to etching.

Example 11 Acid-Etching (3)

A transparent conductor sheet was prepared according to the methoddescribed in Example 7 (e.g., Formula 1). A nanowire network layer wasformed in a matrix layer of about 140 nm thick. The sheet wassurface-conductive having a surface resistivity of about 500 Ω/□.

A region of the conductive surface area of the sheet was then protectedby an overcoat of about 1 μm thick. The nanowires were fully covered bythe overcoat and no surface conductivity was detectable in this region.The region without the overcoat remained surface conductive.

The entire sheet was then dipped into an acid etching solution (20 ppmKMnO₄, 1% HNO₃, 1% NaNO₃) for a minute. The sheet was removed and rinsedwith water and dried under N₂ flow. The entire sheet becamenon-conductive.

In the uncoated area, as shown in FIG. 38B, silver nanowires weredissolved into soluble silver salt despite the presence of the matrix.This result indicates that higher concentration of KMnO₄ (20 ppmcompared to 10 ppm and 5 ppm in Examples 9 and 10, respectively) cancause the dissolution of the nanowires within the matrix. The uncoatedarea was no longer conductive because of the corruption of the nanowirenetwork layer.

The silver nanowires in the overcoat-protected area were unaffected bythe etching. Although no surface conductivity was possible due to thethickness of the overcoat, a nanowire network layer remained underneaththe overcoat (FIG. 38A).

FIGS. 38A and 38B illustrate that the transparent conductor describedherein can be patterned according to standard lithography method using amask, which acts as the thick overcoat as illustrated herein.

Example 12 Color Filter Coatings

Commercial color filters were coated directly with conductive nanowirefilms.

Sample 1 was a chrome black matrix with color filters. As shown in FIG.39A, the R, G and B color filters were arranged parallel to each otherand two adjacent color filters were separated from one another by agroove 780, which prevents light transmission between adjacent pixels.Sample 1 was surface conductive in the direction of the grooves (due tothe metallic content of the black matrix) and registered a sheetresistance of about 40 Ω/□. The sample was not surface conductive acrossthe grooves.

Nanowires were coated directly on the entire top surface of Sample 1.The coated sample remained surface conductive in the direction along thegrooves and showed a surface resistance of about 33 Ω/□. In addition,because the nanowires were coated conformally, (see, FIG. 39A), thecoated sample was also surface conductive across the grooves 780 andshowed a surface resistance of about 100 Ω/□.

The nanowire coating had little effect on the optical properties ofSample 1. The light transmittance was 18.2% in the coated sample,compared to 19.2% in the un-coated sample. The coated sample wasslightly hazier than the plain sample. The haze in the coated sampleincreased from about 2.7% of the plain sample to about 3.5-5.3%.

Sample 2 was an organic resin black matrix with color filters having asimilar arrangement as in Sample 1. Sample 2 (see, FIG. 39B) wasentirely non-surface conductive.

FIG. 39B also shows Sample 2 coated with a nanowire layer. The coatedSample 2 became surface conductive in the direction along the grooves782 and showed a surface resistance of about 56-76 Ω/□. In addition,because the nanowires conformed to the grooves 782, the coated samplewas also surface conductive across the grooves and showed a surfaceresistance of about 61 Ω/□. Overall, a bulk surface resistance wasmeasured at about 120-130 Ω/□.

Optically, the nanowire coating had little effect to Sample 2. The lighttransmittance was 27.5% in the coated sample, whereas it was 26.9% inthe un-coated sample. The coated sample was slightly hazier than theun-coated sample, increasing from about 1.4% to about 4.8% in haze.

Example 13 Surface Pre-Treatment

Transparent conductor samples were patterned by wet-etching process.Prior to etching, the transparent conductor had been masked according toa pattern (physical mask or photoresist mask) and surface-treated in theunmasked regions. Compared to the untreated sample, surface-treatedtransparent conductors were etched at much higher rate.

Using Physical Mask:

Transparent conductor samples were prepared by first forming a silvernanowire film by spin-coating (or other deposition method) on substratesincluding Polycarbonate, glass or PET, with desired transparency andconductivity. The silver nanowire film was subsequently coated withAddison Clear Wave AC YC-5619 hard coat (by spin-coating). The hard coatmaterial was baked and fully UV cured.

A mask (e.g., a plastic film) having a desired pattern was placed on andin contact with the hard coat of the transparent conductor sample. Themask defined regions (unmasked) to be etched. The unmasked region wassubmitted to O₂ plasma or 10 minute UV ozone treatment for 10 minutes.

The mask was removed and the sample was immersed in 100% Transene AgEtchant Type TFS for 10 seconds before it was removed and rinsed in DIand air-dried.

Using Photoresist Mask:

Instead of a physical mask, a photoresist material can be spin-coated onthe silver nanowire film (with a hard coat). When exposed to UV lightaccording to a desired pattern, the photoresist material cures into amask. The transparent conductor samples can be surface treated andetched following the process described above.

Results:

As shown, Samples 1 and 2, which were pretreated by oxygen plasma and UVozone respectively, became non-conductive (infinite resistivity) within10 seconds of etching. In comparison, untreated Sample 3 remainedconductive after 6 minutes of etching.

Sample 4 illustrates a process of invisible patterning in which adiluted etchant was used. Following an oxygen plasma treatment,patterning using 5% Transene Ag Etchant Type TFS created a patternedtransparent conductor with substantially uniform optical properties. Itis believed that the diluted etchant has rendered transparent conductorfilm non-conductive without completely removing the nanowires.

Sample 1—Pretreated with Oxygen Plasma

Total etch time Initial 10 sec O₂ Film Resistivity (ohm) 66.4 infinitePlasma 2 pt Contact 70 infinite Treated Resistance (ohm) Film %Transparency 87.6 90.0 % Haze 2.33 1.10Sample 2—Pretreated with UV Ozone

Total etch time Initial 10 sec UV Film Resistivity (ohm) 63.5 infiniteOzone 2 pt Contact 60 1100 Treated Resistance (ohm) Film % Transparency87.8 90.2 % Haze 2.33 1.10

Sample 3—Untreated

Total etch time Initial 10 sec 30 sec 60 sec 3 min 6 min Untreated FilmResistivity (ohm) 57.4 90.9 110.2 130.5 234.1 473.9 Film 2 pt Contact 701100 1000 1000 6000 1M Resistance (ohm) % Transparency 87.7 87.9 87.988.0 88.1 88.2 % Haze 2.28 2.16 2.09 2.08 2.00 1.95

Sample 4—Dilute Etchant

Total etch time Initial O₂ Plasma 10 sec 100 sec 5% Ag Film Resistivity(ohm) 55 65.9 Infinite Infinite Etchant 2 pt Contact 60 65 InfiniteInfinite Solution Resistance (ohm) % Transparency 87.6 87.3 87.8 88.0 %Haze 2.71 2.84 4.93 4.40

Example 14 Low-Visibility Patterning

A suspension of HPMC, silver nanowires and water was prepared. Thesuspension was spin-coated on a glass substrate to form a thinconductive film of silver nanowires in a HPMC matrix. The conductivelayer was optically clear, with an optical transmission (% T) of about88.1% and haze (% H) of about 2.85%. The conductive layer is also highlysurface-conductive with a surface resistivity of about 25 Ω/□.

Thereafter, a region of the conductive film was treated with anoxidizing agent, e.g., a bleach solution having 0.5% hypochlorite, for 2minutes. The treated film was then rinsed with water and dried in anitrogen atmosphere. The treated region of the film showed substantiallythe same transmission (89.1% T) and haze (5.85% H) as compared to theoptical properties of the untreated region. FIG. 40A shows that thetreated region 820 and the untreated region 822 are visually uniform.

The surface resistivity of the treated region, however, has increased byseveral orders of magnitude and become effectively insulating. Furthermagnification (100×, dark field) of FIG. 40A shows that the silvernanowires were broken or were likely to have been converted to aninsoluble and insulating silver salt such as silver chloride (see, e.g.,FIG. 40B.)

As a comparison, FIG. 41 shows a silver nanowire-based conductive filmthat was treated with a stronger and more concentrated oxidizing agent:30% hydrogen peroxide. As shown, in the treated region 824, nearly allof the nanowires and the organic HPMC matrix were dissolved. The opticalproperties in the treated region 824 and the untreated region 826 arenotably different.

Example 15

A suspension of HPMC, silver nanowires and water was prepared. Thesuspension was spin-coated on a glass substrate to form a thinconductive film of silver nanowires in a HPMC matrix. The conductivelayer was optically clear, with an optical transmission (% T) of about89.1%, haze (% H) of about 3.02% and surface resistivity of about 45Ω/□.

A region of the transparent conductor was soaked in a TCNQ solution withacetonitrile (ACN)(0.5 mg/ml) for different amounts of time, rinsed anddried in a nitrogen atmosphere. Table 5 below shows how thetransparency, haze and resistivity of the region of the transparentconductor exposed to the TCNQ solution varied with amount of timeexposed.

TABLE 5 Before Soaking 10 sec. 20 sec. 60 sec. % T 89.1 89.3 90.0 91.3 %H 3.02 2.36 1.74 0.53 Ohm/Sq. 45 112 1700 Open Circuit

As shown in Table 5, the change in resistivity of a treated region, withrelatively little change in optical characteristics, can be controlledby changing the amount of time treated region is exposed.

Example 16

The change in resistivity of a treated region may also be controlleddepending on the chemical used to treat the region of alteredresistivity. A transparent conducting sample was prepared as describedabove in example 15. A region of the sample was soaked in a solution ofPd(AcO)₂ and ACN (1 mg/mL) for varying amounts of time. The sample wasthen rinsed twice with ACN and dried in a nitrogen atmosphere. Table 6below shows the change in optical characteristics (transparency andhaze) and resistivity as a function of the amount of time the sample wasexposed to the solution.

TABLE 6 Before Soaking 1 min. 11 min. 35 min. 66 min. % T 89.5 89.4 89.288.7 88.5 % H 2.80 2.82 2.81 2.66 2.56 Ohm/Sq. 51 47 58 173 193

As illustrated by a comparison of Tables 5 and 6, the amount theresistivity of the exposed region changes with time can vary dependingon the chemical the region is exposed to.

Example 17 Photoresist Patterning Method

A silver nanowire dispersion was prepared consisting of 0.2% HPMC, 250ppm TritonX100 and silver nanowires. The dispersion was spin-coated ontoa substrate and baked for 90 seconds at 180° C. This nanowire film wasthen spin-coated with AZ-3330F photoresist to make a 2.5 μm transparentconducting film. The transparent conductor was then baked at 110° C. for60 seconds. A photomask was placed in contact with a portion of thephotoresist layer and the transparent conductor was exposed to light for20 seconds at 12 mW/cm². The conductor was then baked for 60 seconds at110° C.

The photoresist was then developed with AZ300MIF developer, rinsed andspun dry. The conductor was then exposed to Transene silver etchant for10 seconds, rinsed and spun dry. The photoresist was then stripped usingAcetone. The transparent conductor was overcoated with Polyset PCX35-39Bat 2.5% dilution in PGME and then cured for 45 min. at 180° C. Theresulting patterned transparent conductor had a line-width of from 5 μmto 10 μm. Larger pattern line-widths have also been obtained usingphotoresist and other patterning methods disclosed herein. For example,line-widths from 10 μm to 300 μm and 10 μm to 50 μm have been obtained.

Example 18 Low-Visibility Patterning by Copper Chloride Etchant

An etchant solution was prepared by mixing 240 g of CuCl₂.2H₂O with 180g of concentrated HCl (37% w/w) and 580 g of water. The finalconcentration of CuCl₂ is about 19% and HCl is 6.8%.

A conductive film of silver nanowires was prepared according to themethods described herein (e.g., Example 17). The conductive film wasetched, and the unetched region and etched region are shown in FIGS. 42Aand 42B, respectively. It could be observed that the two regions showlittle difference in optical properties, yet the etched region is lessconductive and has a resistivity of about 20,000 Ω/sq.

Example 19 Low-Visibility Patterning Etching by Heating

Example 19 demonstrates creating a low-visibility pattern in aconductive film by combining a partial etching step and a subsequentheating step. As discussed herein, the heating completes the etching byfurther rendering the etched region non-conductive or less conductive.

Table 7 shows that a heating step alone actually increases the filmconductivity. In Trials A and B, conductive films (or samples) wereheated for five and thirty minutes, respectively, and their sheetresistivities (R_(s)) were reduced by between 5 and 10 percent.

TABLE 7 Heating Step Trial A R_(s) (t = 0) R_(s) (t = 5 min at 130° C.)106 Ω/sq 94.5 Ω/sq Trial B R_(s) (t = 0) R_(s) (t = 30 min at 130° C.)106 Ω/sq 98.5 Ω/sq

Table 8 shows the effect of heating on a partially etched sample. In thethree trials listed, the samples were chemically etched using a CuCl₂etchant (as described in Example 18) until their sheet resistivity wasapproximately 1000 Ω/sq. They were then heated for either five minutesbut also as little as one minute at 130° C. In each trial, the heatingstep was sufficient to render the sample non-conductive. In other words,the damage of the nanowire network that was initially caused by theetching process was completed by the heating process.

TABLE 8 Etching Step Heating Step Trial C R_(s) (t = 0) R_(s) (t = 90 sin etch bath R_(s) (t = 5 min at 130° C.) at roughly 36° C.) 106 Ω/sq1000 Ω/sq Not conductive Trial D R_(s) (t = 0) R_(s) (t = 70 s in etchbath R_(s) (t = 5 min at 130° C.) at roughly 40° C.) 121 Ω/sq 1200 Ω/sqNot conductive Trial E R_(s) (t = 0) R_(s) (t = 120 s in etch bath R_(s)(t = 1 min at 130° C.) at roughly 32° C.) 121 Ω/sq 1000 Ω/sq Notconductive

Table 9 shows that if the initial chemical etching step is insufficient,i.e., the damage to the nanowires is insufficient; it becomes difficultto make the samples non-conductive even with a subsequent heating step.In Trial F, a sample was etched until its resistance had changed from108 to 120 Ω/sq. Subsequent heating for one minute at 130° C. did notchange the sample resistance. In Trial G, another sample was etcheduntil its resistance had changed from 121 to 198 Ω/sq. Subsequentheating at 130° C. for up to twenty-five minutes did continuallyincrease the sample's resistivity; however, the sheet resistance failedto go beyond 685 Ω/sq. This shows that it is important for the initialpartial etching to cause an etched region to reach a thresholdresistivity (which is indicative of the extent of damage to thenanostructures) in order for the heating step to complete the etching.

TABLE 9 Etching Step Heating Step(s) Trial F R_(s) (t = 0) R_(s) (t = 60s in etch R_(s) (t = 1 min at 130° C.) bath at roughly 33° C.) 108 Ω/sq120 Ω/sq 122 Ω/sq Trial G R_(s) (t = 0) R_(s) (t = 90 s in etch R_(s)(t) at 130° C. bath at roughly 32° C.) 121 Ω/sq 198 Ω/sq t (min) 1 2 515 25 R_(s) 310 340 450 600 685

Table 10 compares the optical properties of two patterned samples: thesample in Trial I was etched chemically (by a CuCl₂ etchant) to benon-conductive and the sample in Trial H was partially etched followedby heating.

In Trial H, an initial partial etching brought the resistivity from 105Ω/sq to 602 Ω/sq, which was sufficient for a subsequent heating step tomake the sample non-conductive. As shown, the final optical propertieswere nearly identical to the initial properties of the sample (prior toetching), i.e., the difference in haze (H %) being about 0.01%, thedifference in transmission (T %) being 0.1%. The sample hadlow-visibility patterns.

In Trial I, the sample was etched to be fully non-conductive. Here,although the transmission remains the same before and after the etching,the haze had decreased by about 0.07%, as compared to the pre-etchinghaze value. The bigger difference between the haze of etched andunetched areas of the film of Trial I (as compared to Trial H) makes theetched areas more visible that those of Trial H.

TABLE 10 Rs % T % H Trial H t = 0 105 Ω/sq 91.8 1.18 Etched to andEtched 180 s at 30° C. 602 Ω/sq 91.7 1.27 Then Heated to Then Heated 90s at Not Conductive 91.7 1.19 Non-Conductivity 130° C. Trial I t = 0 103Ω/sq 91.8 1.15 Etched to Etched 180 s at 32° C. Not Conductive 91.8 1.08Non-Conductivity

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A process comprising: forming a conductive film comprising aplurality of interconnecting nanostructures; and etching the conductivefilm according to a pattern to provide (1) an unetched region having afirst resistivity, a first transmission and a first haze and (2) anetched region having a second resistivity, a second transmission and asecond haze; and wherein the etched region is less conductive than theunetched region, a ratio of the first resistivity over the secondresistivity is at least 1000; the first transmission differs from thesecond transmission by less than 5%; and the first haze differs from thesecond haze by less than 0.5%.
 2. The process of claim 1 wherein theetching comprises contacting the conductive film with an etchantsolution, wherein the etchant solution includes a metal salt.
 3. Theprocess of claim 1 wherein the etchant solution comprises one or moremetal salts selected from copper (II) chloride and iron (III) chloride.4. The process of claim 1 wherein a ratio of the first resistivity overthe second resistivity is at least 10⁴; the first transmission differsfrom the second transmission by less than 2%; and the first haze differsfrom the second haze by less than 0.05%.
 5. The process of claim 1wherein the etching comprises severing at least some nanostructures ofthe etched region.
 6. The process of claim 1 further comprising heatingthe conductive film.
 7. A process comprising: forming a conductive filmcomprising a plurality of interconnecting nanostructures; etching theconductive film according to a pattern to provide (1) an unetched regionhaving a first intermediate resistivity, and (2) an etched region havinga second intermediate resistivity, wherein a first ratio of the firstintermediate resistivity over the second intermediate resistivity isless than 1000; and heating the conductive film such that the etchedregion has a first final resistivity and the unetched region has asecond final resistivity, wherein a second ratio of the first finalresistivity over the second final resistivity is at least 1000, andwherein the etched region and the unetched region are optically uniform.8. The process of claim 7 wherein the first ratio is at least 10, andsecond ratio is at least 10⁴.
 9. The process of claim 7 wherein,following the heating step, the unetched region has a first transmissionand a first haze and the etched region has a second transmission and asecond haze; and wherein the first transmission differs from the secondtransmission by less than 5%; and the first haze differs from the secondhaze by less than 0.5%.
 10. The process of claim 7 wherein thenanostructures are silver nanowires.
 11. The process of claim 7 whereinthe etching comprises contacting the conductive film with an etchantsolution, wherein the etchant solution includes a metal salt.
 12. Theprocess of claim 11 wherein the etchant solution comprises one or moremetal salts selected from copper (II) chloride and iron (III) chloride.13. A process comprising: forming a conductive film comprising aplurality of interconnecting nanostructures; and contacting an etchantsolution with the conductive film according to a pattern to provide anunetched region and an etched region, wherein the unetched region andthe etched region have substantially the same optical properties, andwherein the etchant solution is an aqueous solution including a metalsalt.
 14. The process of claim 13 wherein the etchant solution comprisescopper (II) chloride, hydrogen chloride and water.
 15. The process ofclaim 13 wherein the etchant solution comprises iron (III) chloride,hydrogen chloride and water.